The two Clownfish Marlin and his wife, Coral, are admiring their new home in the Great Barrier Reef. They are both protecting their clutch of over 400 eggs that are due to hatch in a few days. Marlin then sees Coral looking worriedly at a barracuda a short distance away. Marlin tells Coral to get back into the anemone but she tries to protect their eggs. The barracuda then attacks them and Marlin attempts to save his wife but the barracuda knocks him out. When he regains consciousness, the barracuda has eaten Coral and all but one of their eggs. He names the last remaining egg Nemo, a name that Coral liked.
The film then moves on to Nemo's first day of school. Nemo has an abnormally small right fin (due to a minor injury to his egg from the barracuda attack) which causes Marlin to not only worry over his swimming ability but also doubt his son’s general capabilities. Once at school, Nemo disobeys his father and sneaks away from the reef towards a boat, resulting in him being captured by a scuba diver.
In search of help, Marlin bumps into Dory, a naïve but good-hearted and optimistic Regal tang with short-term memory loss. While meeting would-be vegetarian sharks Bruce, Anchor and Chum, Marlin discovers a diver's mask that was dropped from the boat, and notices an address written on it. Before he has the chance to read it however, a chaotic struggle against Bruce's organic appetite occurs and shortly after his restoration, the mask is dropped into a trench in the deep sea. During a hazardous struggle with an anglerfish in the trench, Dory realizes she is able to read the address written on the mask, which leads to Sydney, and manages to remember it despite her short-term memory loss. After receiving directions from a large school of moonfish, Marlin and Dory set out to find Sydney. After a struggle with hypnotic jellyfish that nearly sting them to death, they are recovered by a surf cultured sea turtle named Crush, who takes them on the East Australian Current. In the current, Marlin reluctantly shares the details of his journey with a group of young sea turtles, and his story spreads rapidly across the ocean through word of mouth and eventually reaches Sydney.
Meanwhile, Nemo's captor, a dentist, drops him into a fish tank in his office on Sydney Harbour. There, Nemo meets a group of aquarium fish called the "Tank Gang", led by a crafty and ambitious moorish idol named Gill. The "Tank Gang" includes the fishes; Bloat a porcupinefish, Bubbles a Yellow Tang, Jacques a Pacific Cleaner Shrimp and Deb a Blacktail Humbug. The fish are frightened to learn that the dentist plans to give Nemo to his niece, Darla, who is overly-enthusiastic and infamous for killing a fish given to her previously by constantly shaking its bag. In order to avoid the situation, Gill gives Nemo a role in an escape plan, which involves jamming the tank's filter and forcing the dentist to remove the fish from the tank to clean it manually. After a friendly pelican named Nigel visits with news of Marlin's adventure, Nemo succeeds in jamming the filter, but the plan is later backfired when the dentist installs a new high-tech filter.
Upon leaving the East Australian Current, Marlin and Dory become lost in a huge bloom of plankton, and are engulfed by the mouth of a blue whale after swimming in its path. Inside the whale's immense mouth, Dory calmly tries to communicate with the whale. In response, the whale carries them to Sydney Harbour and expels them through its blowhole. They are then met by Nigel, who recognizes Marlin from the stories he has heard and rescues him and Dory from a flock of hungry seagulls by scooping them into his mouth and taking them to the dentist's office. By this time, Darla has arrived and the dentist is prepared to give Nemo to her. Nemo tries to play dead in hopes of saving himself and, at the same time, Nigel arrives. Marlin sees Nemo and mistakes this act for the actual death of his son. After the dentist throws Nigel out, Gill helps Nemo escape into a drain through a sink plug-hole after a chaotic struggle.
Overcome with despair, Marlin leaves Dory and swims back toward his home. Dory then loses her memory and becomes confused, but meets Nemo, who has escaped into the ocean through an underwater drain pipe. Dory's memory is suddenly restored after she reads the word "Sydney", and she guides Nemo to Marlin. After the two joyfully reunite, Dory is caught in a fishing net among a school of grouper. Nemo enters the net with bravery and directs the group to swim downward to break the net, reminiscent of a similar scenario that occurred in the fish tank earlier in the film. The fish, including Dory, succeed and escape. At this point, Marlin is no longer overprotective or doubtful of his son's safety.
Back at the dentist's office, the high-tech filter breaks down and The Tank Gang have escaped into the harbor, but realize they are confined to the bags of water that the dentist put them into when cleaning the tank.
追问能不能再简短些