In an anemone under the sea, two clown fish Marlin and Coral are about to become parents to hundreds of new offspring. Unfortunately the family is attacked by a much larger fish and only Marlin and one egg, Nemo survive. This makes Marlin over protective of Nemo. Nemo heads off on his first day of school despite Marlin's worries and joins Mr Ray's class. As they head off, the other parents congratulate Marlin on his calmness but let slip the class is headed for the drop off and Marlin panics and swims off after the class.
In class, Nemo and his friends have sneaked away from the main group and are daring each other how far they could swim out to sea where a boat is bobbing above the waves. Just as they challenge Nemo, who tells them he can't because his father said it isn't safe, Marlin arrives and tells Nemo he isn't ready to join school yet. Fed up with his father's overprotectiveness, Nemo swims out into the open water and over to the boat. Marlin shouts at him to come back and as Nemo swims back, a diver appears and catches him in a small net. The divers return to the boat and Marlin tries to swim after the it but soon loses it due to the speed of the boat. As he swims onwards trying to locate the boat he runs into a blue fish named Dory, who tells him she knows where the boat went. Marlin follows her as she swims off but after a while she shouts at him for following her. It turns out she forgets things very easily and Marlin decides to leave her, but as he turns around, they come face to face with a huge shark. Dory is friendly towards the shark whilst Marlin is terrified. The shark, Bruce, invites them to a gathering and 'insists' they attend. He leads them through an old minefield where another two sharks, Anchor and Chum, are waiting. Marlin is convinced he's about to get eaten but the gathering turns out to be a self help group to stop the sharks eating fish. During the meeting, Marlin spots one of the divers masks and rushes over to it, trying to make out the marks on it. Dory takes the mask and takes it to the sharks to see if they can read it. marlin pulls at the mask trying to get it back and accidentally hits Dory on the nose. Bruce smells the blood and falls off the wagon, chasing the two fish trying to eat them. Whilst they are trying to escape, Marlin finds out Dory can read. They manage to scare the sharks away by blowing up the mines and escape.
Meanwhile Nemo has been thrown into a fishtank where he meets a group of other aquatic friends, Bloat, Bubbles, Deb/Flo, Peach, Gurgle, Jacques and Gill along with Nigel, a pelican. He finds out he is a birthday gift for the dentists niece Darla, who killed her fish last year by shaking the bag he was in. Gill seems to be hatching some kind of plan and they initiate Nemo into their group and Gill tells them his escape plan which is to block the filter so the dentist takes them out of the tank to clean it and they roll the plastic bags out of the window and into the sea.
Back in the ocean, after following the mask into a dark chasm and being almost eaten by an anglerfish, Dory has read the markings on the mask and she and Marlin are on their way to Sydney to find Nemo. Marlin feels Dory is slowing him down and he tells her he's better off going on without her. She's upset by this and and a group of fish try to cheer her up by imitating fish, including Marlin. He gets fed up and goes to leave but Dory takes pity on him and gets directions to Sydney from the fish. He realises she's helping him and they carry on together. They also warn Dory to go through the upcoming trench and not swim over it. As they approach the trench, Dory tells Marlin they should swim through it, but seeing how dark it is, Marlin tricks her into swimming over it where the water is clear. However as they swim along, Dory gets stung by a tiny jellyfish and they see thousands of bigger ones appear. Realising they can bounce on the tops, they race each other through the jellyfish but Dory gets stung and Marlin pulls her out, getting stung himself and they both fall unconscious.
Back in the tank, Nemo and the gang are carrying out the plan to dirty the tank. Nemo swims into the filter and places the pebble to block it, but as he swims out of the tube, the pebble becomes dislodged and he almost gets sucked into the filter himself. Using long plants, the others pull him out and Gill gives up on the escape plan.
Marlin has regained consciousness and he and Dory are riding with a group of sea turtles towards Sydney. The turtle he is riding, Crush, makes Marlin realise that children have to go their own way at some point. Marlin tells Crush and his son Squirt the story of his adventure so far and the story is passed along through various sea creatures until it reaches Nigel the pelican who tells Nemo his dad is on his way. Finding new hope, Nemo goes back into the filter and sucessfully blocks it this time.
Marlin and Dory have left the turtles and after Dory's attempts to speak whale, they end up inside the whale. The whale tells them to go to the back of his throat but Marlin is still convinced its going to eat them but Dory tells her to trust him. Feeling bad because he didn't trust her about the trench, he trusts her this time and the whale blows them both out of his airhole, into Sydney harbour. A pelican tries to eat them but Marlin blocks their way into his stomach by choking him. Nigel goes to help and dislodges Marlin and Dory and realises who they are and promises to take them to Nemo, avoiding a group of hungry seagulls on the way.
Back in the tank, there has been a problem with the plan as the dentist has replaced the filter with a newer model and the tank is clean again. Darla arrives and chaos breaks out as Nemo plays dead in an attempt to get flushed down the drain. Marlin arrives and sees Nemo and thinks he's really dead but Nemo hears his voice and sits up. The dentist chases Nigel, Dory and Marlin out of the surgery. Darla shakes the bag realising Nemo isn't dead and he ends up on the dentists table. Gill and the fish blast Gill out of the tank and he knocks Nemo down the drain. The dentist puts Gill back in the tank and Darla ends up with no fish.
Nemo gets flushed into the sewers and back into the ocean. Meanwhile Marlin, still thinking Nemo is dead, tells Dory he's going home and leaves her. As she swims around alone, Nemo arrives and they decide to go and find Marlin. They swim off after him and as they find him, Dory gets trapped in a fishing net. Nemo knows how to save her and convinces Marlin to trust him and by telling all the fish to swim down, they break the net and free all the fish, rescuing Dory.
They return to the reef and a much more confident Marlin lets Nemo go back to school. Meanwhile the fish in the tank have managed to carry out the escape plan and rolled their plastic bags into the ocean.
Name: - Nemo
Colour: - Orange with white stripes
Eye Colour: - Brown
Lives: - The ocean
Family: - Father, Marlin; Mother, Coral (deceased)
Known Associates: - Dory, Gill, Bloat, Peach, Gurgle, Flo/Deb, Bubbles, Jacques
Personality:- Nervous, curious, can be brave
Likes: - Learning
Dislikes: - Being told he can't do things
Loves: - None
Enemies: - Darla
Any alter egos? : - Sharkbait
Motto: - I can do this!