Nemo is a young clownfish who lives with his father, Marlin in a sea anemone. Before he hatched from his egg, Nemo's mother, Coral, and the other eggs were killed in an attack by a barracuda. Only Nemo's egg survived, albeit slightly cracked. As a result of the crack, one of Nemo's fins is smaller than the others, thus Marlin worries about him. One day, Nemo heads off to his first day of school. At one point, he boldly leaves the reef to touch a boat on the open ocean. Marlin, who is watching, argues with Nemo, but Nemo is suddenly taken by a diver to a fish tank in a dentist's office in Sydney, Australia.
In the tank, Nemo meets the Tank Gang, and their leader, Gill. The fish are horrified when they learn that Nemo, whom they have dubbed "Sharkbait," is to be a gift for the dentist's niece, Darla. Darla has a past history as a "fish-killer" and so the Tank Gang decide to help Nemo avoid that fate. The initial plan to make the tank dirty fails when the dentist installs a new cleaner. While the plan is being carried out, however, Nemo learns from Nigel the pelican that his father is looking for him, which lifts the young fish's spirits.
When Darla arrives, Nemo is placed in a bag. Nemo is able to fool the dentist by playing dead, causing the dentist to flush Nemo into a drain. However, an observing Marlin believes that Nemo has really died and swims off. In the ocean, Nemo runs into Dory, who has earlier helped Marlin in his search for Nemo. Though Dory does not initially remember Marlin's goal, due to a short-term memory loss issue, she soon recalls it and leads Nemo to Marlin.
Marlin and Nemo are thrilled to see each other, but Dory is suddenly caught in a fisherman's net with a school of grouper. Nemo is able to use the skills he obtained from his time in the tank to save Dory. This gives Marlin a new-found respect for his son's abilities, and Marlin becomes less protective of him. The film ends by showing Nemo going to school.
Nemo appears in several Disney attractions including the Finding Nemo Submarine Voyage at Disneyland, which features scenes from the film. The Seas with Nemo & Friends, located in Epcot at Walt Disney World Resort, also features Nemo. The ride's storyline has various characters from the film searching for Nemo, who has apparently become lost once again.
- The name "Nemo" may have been intended as a reference to Captain Nemo, a major character in Jules Verne's novel 20,000 Leagues Under the Sea and Disney's 1954 film adaptation.