Giacomo and Tommaso are two fishermen, and they are on their way for a late-night fishing trip. Giacomo looks scared and tells Tommaso that they have to go back, because they’re getting too close to an island, where sea monsters are believed to be living.
But Tommaso tells him to stop worrying, because he thinks sea monsters are not real. He also says the stories about sea monsters are just a way to keep them from getting to nice fishing spots.
Giacomo tries to convince him, but Tommaso insists that they’ll be fine and there’s nothing to worry about. He starts playing music from the gramophone, and they both continue moving in the ocean. Unknowingly, a sea monster sneaks up on their boat, and starts stealing some of the things inside. While Giacomo tries to drag out the fishing net from inside the water, he sees the sea monster, and immediately alerts Tommaso, who looks shocked.
As the sea monster tries to escape, it gets stuck in the net, but as both men try to capture it, the sea monster escapes. Seeing this, Tommaso tells him they have to leave before it returns to them.
The next day, another sea monster named Luca, is struggling with looking after all his mother’s fish at the bottom of the ocean. He manages to get all of them together, after which he takes them to where the fish are to feed. While he’s there, he sees one of the fishes bouncing on a clock.
Luca goes closer and is intrigued by what he sees, especially when the clock rings. He also finds a card lying around and picks it up.
Just as he admires the objects, he sees a boat moving on the surface. He shouts for all the fishes to hide, saying that the land monsters are around.
As they hide, Luca imagines himself swimming to the surface, though he’s not able to move out of the water. He comes back to reality when his mom, Daniela, calls him to come home for lunch.
When he gets home, Daniela asks Luca if he was careful to avoid being seen by the land monsters. Luca says he was careful, but Daniela scares him by saying the land monsters are dangerous, and he should try to not get caught.
Shortly after, Luca goes in and greets his grandmother, Paguro, and his dad Lorenzo. As the family is about to have lunch, His grandma sees that Luca looks distracted, so she asks him what he’s thinking about. Luca hesitates at first, but he then asks where the boats come from.
This question shocks Daniela and Lorenzo, but Grandma Paguro tells Luca that the boats come from a land monster town, just above the surface. She also tells him that she has been there before.
Daniela is not happy with Paguro telling Luca about things on the surface, but Paguro says he’s bound to find out one day.
Luca apologizes for his question, saying he was just curious, but Daniela tells him that the curious ones usually get caught.
She then proceeds to warn him that he’s not allowed to think, or even contemplate going to the surface, because it is extremely dangerous. After that, she asks him to return to watching the fish. As Luca is about to leave, she tells him she loves him.
When Luca returns to watch the fish, he sees more interesting things in the water, but as he picks them up to take a look at them, someone starts to sneak up behind him.
Because the person is in a deep-diving suit, Luca is terrified immediately he looks back. He runs to hide, but the person still follows him. As Luca then runs out of options for things to do, the person opens the mask, and it turns out to be another sea monster named Alberto, who tells him not to worry because he’s not a human or land monster, as they call it.
Luca looks relieved, and Alberto proceeds to pick up all the human things in the sea. He says it’s his things, and while he packs them, Luca holds his harpoon for him.
When Alberto is through with taking all his things, he goes to the surface. Luca is shocked to see this, especially because Alberto also transforms into a boy. Before Luca can understand what’s going on, Alberto drags him out, and he also becomes a boy.
However, Luca is terrified of how he looks. Alberto just sits calmly and asks if it’s Luca’s first time.
Luca says it’s his first time, but Alberto tells him to calm down and breathe. Immediately after he does this, Luca is fully able to admire his environment, but he soon returns to the water, saying he’s not supposed to be at the surface.
When Luca gets home, he remains intrigued by how the surface looks, and he even tries to ask his grandmother more questions about it, but she’s already fast asleep.
The next morning, Luca creates a rock version of himself that he calls Smuca. He tells the fish that Smuca will be watching them, while he goes to check the surface. As Luca prepares to get out of the water, he is unable to summon the courage to do it.
Shortly after, Alberto sees him while he’s trying to get some new human things from the water. He tells Luca to come with him, and Luca is finally able to do it.
However, Luca is unable to walk, but Alberto teaches him, and even though he struggles with it a few times, he finally gets it. Alberto then introduces himself properly to Luca, and offers to show Luca some more human things he found.
As they head over to where Alberto stays, Luca is shocked to see it. Alberto says he stays there with his dad, who is not always around, so he gets to spend most of his time alone.
Alberto also tells Luca that all the best things are above the surface. Together, they then move to Alberto’s room, where he has many human things.
Luca looks really amazed by all that he sees, but one thing that catches his attention is a picture of a bike called Vespa. Alberto says it’s the best creation by humans, and Luca can’t help but imagine himself riding one.
Luca then asks Alberto if he’s going to make one, since he has some tools, they can use. Alberto realizes it’s really possible to make one, so he asks Luca to join him in doing that. Luca initially agrees but says he can’t because he needs to return home.
However, the thought of building their own Vespa is so enticing that Luca ends up waiting, until they’ve built one.
When he gets home that day, his parents are furious about why he came home late. As he thinks about what to say, Grandma Paguro saves him and tells Daniela that she sent him to get some sea cucumbers for her, and that’s why he’s late.
With this, Luca is able to escape further questioning, and he looks excited about how the day went.
The next day, Luca goes to meet Alberto again, and they start thinking of trying out their Vespa. While Luca is scared of trying, Alberto takes their makeshift Vespa for a ride.
Despite the Vespa breaking apart before he goes too far, Alberto seems to enjoy the ride. They decide to make another one, and after doing that and crashing it again, they play around and also watch some humans talk while on a fishing trip.
Meanwhile, Daniela discovers that Luca is not watching the fish, and he actually arranged some rocks to make it look like he is still there.
Luca continues to play with Alberto, and they make yet another Vespa. This time, Alberto asks Luca to join him, but when Luca says he can’t because he’s scared, Alberto tells him to get rid of the voice in his head, that keeps telling him he can’t do it.
Luca is able to do this, and he joins Alberto on the ride, which sees both of them eventually fall into the water.
When everywhere starts to get dark, Luca admires the stars in the sky. Alberto says the stars are small fish that go up to the sky to sleep. He also refers to the sun as a big fish that protects the smaller fish.
Even though it doesn’t sound believable, Luca thinks it’s really cool. He also tells Alberto that he’s so cool and living a really interesting life, unlike him, who can only dream of it.
He then sees the human town far ahead, and asks if Alberto has been there before. Alberto says he has, and that his dad also told him a lot about the place. Luca believes the town will have a lot of Vespas, and he tells Alberto it would be nice to have their own real Vespa.
As Luca imagines himself riding a real Vespa, he loses track of time and falls asleep. When he wakes up, it’s already very late, and he just runs home.
He tries to sneak into the house when he gets there, but there’s no escaping for him tonight, as Daniela appears in front of him. Luca then sits at a table with his parents, while his clock and card are on the table.
His father Lorenzo tells him that he’s in big trouble, and has to promise he won’t sneak to the surface again.
But Luca says it’s not that bad above the surface, and he’s always careful with his friend. As they hear that there’s someone else influencing him, Luca’s parents are shocked, and suddenly, his Uncle Ugo appears to show himself in the room.
Luca wonders what Uncle Ugo is there to do at that time of the night, and Daniela tells him that he’s going to stay with his uncle at the bottom of the ocean.
Luca is unhappy about this, but Daniela tells him that her mind is made up, and he has to do as he’s told. As Daniela tries to tell him that she loves him, Luca leaves angrily, but instead of going to his room, he goes back to the surface.
Luca tells Alberto about his parents’ plans, to send him to the bottom of the ocean. Even though Alberto offers Luca the chance to stay with him on the island, Luca thinks it’s not safe, because his parents will easily find him. Alberto then suggests that they go to the human town.
Luca thinks it’s too dangerous at first, but when Alberto says they will be able to get a real Vespa there, Luca agrees, and together, they head over to the human town called Portorosso.
As they get to the town, Alberto tells Luca to follow his lead and avoid getting wet. Luca starts admiring the town as they walk around, but he soon sees some indication that the town is filled with sea monster hunters. He tells Alberto that they have to leave, but he seems to throw away any thought of leaving, as soon as he sees a man named Ercole riding a Vespa.
After Ercole parks his Vespa, Luca and Alberto admire it with full interest. Just then, one of the town’s kids kicks a ball in their path, and asks for it to be kicked back.
As Luca kicks it, his shot is too strong, and it ends up hitting Ercole’s Vespa, though one of Ercole’s minions, named Ciccio, stops it from falling to the ground.
But Ercole doesn’t like what just happened, and he finds out who did it. He approaches Luca and Alberto, and figures out that they’re new to the town. Ercole jokes about their clothes, and also teases them about how they look.
He then introduces himself, and calls himself a five-time winner of the Portorosso Cup. He also says the prize money from the competition was what he used to buy his Vespa.
Ercole soon says he doesn’t like how Luca is smelling. Alberto defends Luca, but Ercole asks Ciccio to hold Alberto while he gives Luca a bath. Luca is beyond terrified as this will expose him to be a sea monster, but there’s nothing he can do.
Just as Ercole is about to push Luca’s face into the water at the fountain, a girl named Giulia stops him.
She tells Ercole that his reign of terror will soon come to an end. But Ercole makes fun of Giulia, saying she’s no match for him, as he’ll continue to win the Portorosso Cup.
When Ercole then says she needs to leave, so that he can have fun with Luca and Alberto, Giulia says she’s leaving with them. She offers them a ride, and together, Luca and Alberto join her as she leaves.
Just then, a police officer speaks to a man about setting up a huge reward, for anyone who can find a sea monster. Ercole looks excited at this challenge, as he loves to win prizes, and he tells Ciccio to get a harpoon so they can start looking.
Meanwhile, Giulia laments about Ercole’s habit of making fun of her. She also tells Luca and Alberto that she helped them, because they were underdogs just like her.
Luca and Alberto don’t understand what she means by underdogs, but Giulia tells them it’s a word for kids like them who are different. She then asks if they’re in town for the Portorosso cup, but when they don’t respond, Giulia greets them goodbye and leaves.
However, Luca gets curious about the cup, and he tells Alberto to follow Giulia and ask about it, because that’s how Ercole said he was able to buy his Vespa.
Alberto proceeds to ask Giulia about the cup, and Giulia tells them they can get the prize money if they win the cup. Luca presses further and asks if they can use the prize money to get a Vespa, and she tells them it’s possible.
Luca and Alberto look really happy about this, and they tell Giulia they’ll try to win.
Giulia tells them it won’t be easy, because she has been trying to beat Ercole for a while without success. Seeing this, Luca suggests to Alberto that they join Giulia’s team.
After a brief hesitation, Giulia agrees, and she tells them to show her what they can do. Luca is able to learn how to ride a bicycle, but he’s still not good enough. As the competition is a triathlon that also involves swimming and eating, Giulia asks Alberto if he can swim.
Knowing what will happen if he tries it, Alberto says he’s bad at swimming. With that, Giulia says she’ll take the swimming part, while Luca will take the cycling aspect, and Alberto will compete in the eating category.
Giulia then says she needs to get the money for the entry fee from her father, Massimo. She takes Luca and Alberto home to see her father, and the boys look terrified to see how fierce Massimo looks, and how harpoons are on the walls, too.
Massimo shows Giulia a picture of the sea monster that was recently sighted, and while Giulia thinks it’s a fake picture, Massimo says he’ll try to capture it, because he knows sea monsters are real.
The way he uses his knife to clip the picture to the wall, is so terrifying that Luca spits the water in his mouth on Alberto. This changes part of Alberto’s face to a sea monster, though they’re able to hide before Massimo or Giulia sees them.
Later that night, Massimo tells Giulia that he doesn’t want her to participate in the cup, because she’s always upset, and he doesn’t have enough money to pay for the entry fee, because he has been struggling to catch fish in the ocean. Luca and Alberto volunteer to help Massimo get more fish, and he agrees.
That night, when Luca and Alberto are leaving, Giulia figures out that they have nowhere to stay, so she lets them stay in her tree house. While there, Luca and Alberto talk about how interesting their day was, and how they’re blending in quite well.
In the morning, though, both of them wake up as sea monsters due to the rain that fell on the previous night. They are, however, very quick to avoid getting seen.
Both boys then join Massimo on a fishing trip, and they show him exactly where he can get more fish. This works out just fine, and Massimo gives them the money for the entry fee.
After signing up for the cup, Luca, Alberto, and Giulia start preparing for their parts in the competition. While Giulia is swimming, and Luca and Alberto are watching her from inside a boat, Ercole tries to crash into them.
In the process, water touches Alberto, and he turns into a sea monster, but he’s able to hide inside the ocean. Ercole looks suspicious when he doesn’t see Alberto in the boat again, but before he can realize what is happening, Alberto becomes human again and gets into the boat.
Elsewhere, Daniela and Lorenzo arrive at Portorosso to look for Luca, after feeling guilty for how they tried to push him away.
To find Luca, they start pouring water on the kids in the town, but none of them seem to be their son. As they lament about where Luca could be, Luca sees them but immediately goes into Giulia’s house.
That night, Luca tells Alberto that he might’ve seen his parents. Alberto, however, says it’s not possible, because his parents would never come to a human town.
As they’re talking, Giulia joins them and talks about how their training went well. Soon after, Massimo asks Alberto to come and help him with his nets, but as Alberto leaves, he looks unhappy about how he might be losing his friend to Giulia.
When they’re alone, Giulia tells Luca about what winning the Portorosso Cup means to her, as the whole town will cheer for her, and she’ll also be able to finally beat Ercole.
As they continue talking, they stare at the stars, and when Luca calls them small fish, Giulia tells him what they really are. She also takes him to a place, where he uses a telescope to look at the stars closer.
Luca is intrigued by what he sees, and when they get home, Giulia shows him more books about the solar system, and tells her that her school has a bigger telescope. Before he leaves her room that night, Giulia gifts Luca a book on the solar system.
Luca shares his excitement with Alberto later that night, but Alberto doesn’t seem to like how he keeps mentioning things Giulia told him.
Alberto then gets uncomfortable when Luca talks about going to Giulia’s school, to take a look at the big telescope they have. He says it’s very dangerous, and things won’t turn out fine if they notice that he’s a sea monster.
The next day, training continues for the trio, and during Luca’s training, he sees his parents again. This time around, they see him, too, but Luca is able to move really fast to stop them from catching him.
When it’s time to ride down a hill, Luca stops, and Giulia shows him a train that goes to the city where her school is.
Luca asks if her school is for everyone, and when she says anyone can go there, as long as they pay a fee, Alberto doesn’t look happy. He tells Luca to continue with his training, but Luca says he’s scared to proceed.
Giulia then tries to offer him some encouragement, but Alberto says she needs to stop bossing him, and he knows what Luca needs. He hops on the bicycle and starts riding dangerously with Luca, who remains scared.
The boys eventually fall into the ocean, and to make sure Giulia doesn’t see them as sea monsters, they take another route out of the water.
Alberto apologizes for what he did, but Luca doesn’t sound happy at all. When Luca also mentions that he saw his parents earlier, Alberto says it’s not possible, and the town is making him lose his mind.
Luca doesn’t agree with this, and he tells Alberto that he wants to go to school. The two of them start fighting, but before things get out of hand, Giulia shows up.
She senses that something is wrong between them, but Luca says nothing is wrong, and they only want to know if they can come to her school now.
Giulia likes this, and she says it’s a very good idea. However, Alberto asks Giulia if the school is open to everyone, including sea monsters. Giulia doesn’t understand what he’s saying, but Alberto suddenly dives into the ocean and shows that he’s a sea monster.
Giulia immediately runs back, and asks that Alberto doesn’t hurt her and Luca. Alberto tries to prove his point to Luca, that Giulia will only see him as a monster.
But Luca calls out Alberto as a sea monster, and this shocks him. He then looks sad as he has to leave before Ercole can catch him.
That night, when Luca and Giulia return home, Massimo asks for Alberto, and when they say he’s gone, Massimo insists that he’ll go ahead and look for him. After Massimo leaves, Giulia pours water on Luca to confirm her suspicions, and she realizes that he’s also a sea monster.
She suddenly looks confused, as she doesn’t understand why he had to risk his life just to get a Vespa.
Luca says they actually came to Portorosso because his parents were going to send him away. Giulia says it won’t be safe for him to stay again, because if the people in the town find him, they’ll kill him.
Luca then says he has got to leave. After sitting for a while and watching the waves of the ocean, Luca decides to go and see Alberto.
When he gets to Alberto’s room on the island, he sees that Alberto has scattered everywhere. He soon sees Alberto, who says Luca shouldn’t have come. Luca apologizes for what he did, but Alberto says he’s always the problem, and Luca is better off without him.
He also mentions that his dad left him long ago, and he has been staying alone since then. Alberto insists that Luca leaves, but Luca says he’ll go and win the cup and get them the Vespa, so they can ride away together.
Luca sounds excited about his plan, and he tells Alberto he’ll be back after he fixes everything.
At the Portorosso cup the next day, Luca opts to split up the team, and when Giulia finds out that he’s around and ready to participate alone, she looks very confused and wonders how he’s going to swim.
Luca arrives for the swimming competition in Alberto’s deep-dive suit, and while Ercole jokes about how Luca looks, Giulia tells him that trying to swim is a very bad idea.
The competition starts, and Luca is able to perform well, just like Giulia, though Ciccio, who represents Ercole in the swimming competition, struggles and ends up in last place.
For the eating competition, Giulia is the first to finish, and Ercole forces his participant, Guido, to finish next, after which he is set to continue the last part, which is the cycling competition.
Luca also finishes his food, and he gets on his bike to join the race soon after. He is able to move really fast, and even get away from his parents, who have found him again. He soon overtakes Ercole, but rain starts falling, and he has to stop under a shed.
As he wonders what to do next, Alberto shows up with an umbrella, but it seems too late as Ercole overtakes Luca and kicks Alberto away.
Alberto is exposed to the rain and transforms into a sea monster. Ercole stops riding and plans to capture Alberto, so that he gets a prize. As Ercole uses his net to catch Alberto, Luca rides into the rain and saves his friend. Luca rides really fast to escape to the ocean, but Ercole catches up to them.
However, just as he stands on his bicycle to get a clear shot to kill the boys, Giulia intentionally crashes into him, and they both fall.
Meanwhile, Luca and Alberto are able to move further forward before stopping to check on Giulia. Massimo gets his harpoon and is about to attack the sea monsters, when he realizes that it’s Luca and Alberto.
As the fishermen in the town surround the boys to kill them, Ercole shows up and says he’s the one who captured them, so he’s the one who can kill them and get the prize.
However, Giulia defends Luca and Alberto, saying they’re not monsters. Massimo also says the same, and he announces them as the winners.
Ercole protests this, but since Luca’s bicycle really crossed the line, the competition’s judge pronounces them as the winners of the Portorosso Cup. Daniela and Lorenzo also expose themselves in the rain, so they can hug Luca and tell him how much they love him.
Soon after, Luca and Alberto buy their Vespa, and when they get to Giulia’s house, she asks what they’ll do next. Alberto says they’ll stick around for a while until they’ve fixed the Vespa, to be able to take them around the world.
Just then, Giulia says she’s got to pack for school, and as Luca continues to sound interested in the things she’ll learn at school, Giulia offers to give him some books. As they leave, Alberto takes a look at the Vespa, and wonders if it was worth it.
The next day, when Giulia is leaving, Alberto gives Luca a ticket to go with her, saying he realized they don’t need the Vespa and sold it back.
Luca is excited about this, and he proceeds to ask his parents, who give their blessings, and they tell him that Alberto already convinced them.
Luca asks what Alberto will do after he leaves, and he says Massimo wants him around, so he’s staying. When Luca asks how he’ll remember Alberto, his friend gives him a drawing they made, when they still had dreams of riding around the world in a Vespa.
They hug each other, and Alberto thanks Luca for getting him off the Island.
Luca then gets on the train, and as it moves, Alberto continues to shout his name and cheer loudly for him.