Marlin is a timid and neurotic clown-fish who is extremely protective of his only son Nemo. At the beginning of the film, Marlin and his wife Coral are making a new home together at the edge of the reef. They have just laid their eggs, when a hungry barracuda attacks, killing Coral and the children. Only one egg survives, Nemo, who Marlin promises to protect forever.
When Nemo is stolen from his underwater home, Marlin must swallow his fear of the perilous ocean and travel across the Pacific to Australia to find his son.
Nemo adores his father but feels smothered by his over protectiveness. He desperately wants to explore the sea in its entirety, but his father's fears of the ocean prevent him from doing so.
Nemo has one fin that is smaller than the other, making him prone to swimming off-balance. On the first day of school, Nemo becomes enraged when his father turns up during a trip to the drop-off point. To prove that he does not need coddling, he tries to impress by swimming out in to the open sea but he is taken by a scuba diver.
Dory is a blue tang fish, suffering from short-term memory loss. Dory swims happily but absent-mindedly through life. She joins Marlin in his quest to find his son -- much to his annoyance!
Dory is naive; she spies a small jellyfish and lovingly calls him her "little squishy" before being stung. She also has elements of intelligence deemed by the fact she can read. Dory's sunny point of view and optimism rubs off on Marlin. Her "just keep swimming" attitude teaches Marlin that danger is best faced and out-maneuvered, not hidden from.
Bruce is an Australian Shark and the leader of the group called Fish Friendly Sharks. He is determined to change the way that sharks are viewed and makes a pledge, with two of his other sharks friends Anchor and Chum, that "fish are friends, not food."
Bruce invites Marlin and Dory to a 'get-together' with his group. Bruce is an inspiration to the other sharks after stating that it has been 'three weeks since his last fish.' Bruce later has to be restrained when Dory cuts herself; he loses his self-control after a whiff of her blood.
On his first day of school, Nemo makes friends with Tad, a yellow longnose butterflyfish, Pearl, a flapjack octopus who squirts ink when she is nervous and Sheldon, a seahorse who is water intolerant. Nemo rolls his eyes at his father when he gently tells the juveniles that Nemo has a "lucky fin." However, Pearl shares with Nemo that she has one tentacle shorter than the others, which lightens Nemo's mood.
The children delight in seeing their science teacher, Mr Ray, a spotted eagle ray. Mr Ray claims that, "knowledge exploring is oh so lyrical." He teaches his pupils by taking them out on the reef and singing memory-aiding tunes.
When Nemo is captured, he is transported to a fish tank in the Sydney office of dentist P. Sherman. He meets the "tank gang;" the other aquarium captives including Jacques, a pacific cleaner shrimp, Deb, a four striped damsel fish, Peach, a pink starfish, Bloat, a pufferfish, Gurgle, a royal gramma, Bubbles, a yellow surgeonfish and their gruff leader, Gill an Idol fish
Similar to Nemo, Gill has a damaged fin and goes all out to teach Nemo an important message; a handicap need not impede him.
Whilst riding on the East Australian Current into Sydney, Marlin and Dory befriend a sea turtle called Crush and his children, including one called Squirt. The youngsters are fascinated by Marlin, who they call 'jellyman,' and the tale of his epic and difficult journey; braving sharks, a stinging jellyfish forest and hungry seagulls. Dory is equally astonished, who has forgotten she too has shared all these encounters.
Marlin learns that Crush is 150 years old. He is keen to remember this as he knows Nemo will be impressed.