Nemo, a clownfish, is excited for his first day of school, and his protective father Marlin accompanies him. While on a field trip, Nemo spots a speedboat in the distance and decides to swim up to it. He is captured by a diver and taken on board the speedboat, which then departs. Marlin chases after the speedboat, but soon loses it. After noticing a diver's mask fall into the water, Marlin chases it down, but is unsuccessful in retrieving it.
Marlin meets Dory, a blue tang who suffers from short-term memory loss. She tries to help lead Marlin to the boat, but soon forgets what they are doing. They meet a shark named Bruce, who is fish-friendly. Bruce invites them to a party inside a sunken submarine, although Marlin is sceptical of Bruce. In the submarine, Marlin finds the mask that he tried to catch earlier. Dory gets a nosebleed after an argument with Marlin, and Bruce becomes violent after smelling the blood. He tries to eat Marlin and Dory, forcing them to flee the submarine.
Meanwhile, Nemo is placed in a fish tank in a dentist's office, and he quickly befriends the other fish in the tank. In the ocean, Dory reads an address on the mask, which points her and Marlin to the city of Sydney. Upon arriving there, the two fish meet Nigel, a pelican who agrees to take them to the dentist's office. While they are en route, the dentist puts Nemo in a bag to give to his niece. Marlin, Dory, and Nigel arrive at the office, but must depart as Nigel is forced out by the dentist. Gill, a fish in the tank, helps Nemo escape via the dentist's sink.
Nemo travels his way through the sewers and into the ocean, where he manages to find Marlin and Dory. Shortly after, a fishing net catches Dory and several other fish and tries to pull them out of the water. Nemo comes up with an idea to get all the fish to swim down. This successfully breaks the net and frees them.
After their adventure, Marlin is not as protective of his son, knowing he can look after himself. Dory also spends a lot of time with the two, and she accompanies Marlin as he takes Nemo to school.