Two clownfish, Marlin and Coral, admire their new home in the Great Barrier Reef and their clutch of eggs when a barracuda attacks, knocking Marlin unconscious. He wakes up to find that Coral and all but one of the eggs have been eaten by the barracuda. Marlin names this last egg Nemo, a name that Coral liked.
Following his birth, Nemo develops a smaller right fin due to damage to his egg, which limits his swimming ability. Worried about Nemo’s safety, Marlin embarrasses Nemo during a school field trip. Nemo disobeys his father and sneaks away from the reef towards a boat, and is captured by scuba divers. As the boat departs, a diver accidentally knocks his diving mask overboard. While attempting to save Nemo, Marlin meets Dory, a good-hearted and optimistic regal blue tang with short-term memory loss. Marlin and Dory meet three Australian sharks – Bruce, Anchor and Chum – who claim to be vegetarians. Marlin discovers the diver’s mask and notices an address written on it. When he argues with Dory and accidentally gives her a nosebleed, the blood causes Bruce to lose control of himself and enter a violent feeding frenzy. The pair escape from Bruce, but the mask falls into a trench in the deep sea.
During a hazardous struggle with an Anglerfish in the trench, Dory sees the diving mask and reads the address located at 42 Wallaby Way, Sydney. As Dory finishes reading the address on the mask, Marlin is able to trap the Anglerfish using the eyepiece of the mask. To her own disbelief, Dory is able to remember the address on her own despite her short-term memory loss. The pair swims on, receiving directions to Sydney from a school of moonfish. Marlin and Dory encounter a bloom of jellyfish that nearly kills them. Marlin loses consciousness and wakes up on a sea turtle named Crush, who takes Marlin and Dory on the East Australian Current. Marlin tells the details of his long journey with a group of sea turtles, and his story is spread across the ocean. He also sees how Crush gets along well with his son Squirt.