Nemo is a young clownfish who lives with his father Marlin in an anemone in the Great Barrier Reef. Nemo, despite being hampered by a lame right fin, is eager to explore life around the ocean. Marlin, however, is overprotective of him, having lost his wife Coral and all their other eggs in a barracuda attack, leaving Nemo as his only child. On Nemo's first day of school, Marlin unintentionally embarrasses him, and while Marlin is distracted with the teacher, Mr. Ray, Nemo defiantly sneaks away from the reef toward a speedboat, where he is captured by a pair of scuba divers. Marlin tries to chase the boat and meets Dory, a blue tang who suffers from acute short-term memory loss, who offers her help. During an encounter with three sharks who have sworn to abstain from eating other fish, Marlin notices a diver's mask that fell from the boat that took Nemo. Marlin and Dory fight over the mask, giving Dory a nose bleed, which sends one of the sharks into a feeding frenzy, and Marlin and Dory narrowly escape, but are knocked unconscious when the sharks accidentally set off a ring of old naval mines.
Meanwhile, Nemo is placed in an aquarium in a dentist's office in Sydney, Australia, where he meets the Tank Gang, including yellow tang Bubbles, starfish Peach, cleaner shrimp Jacques, blowfish Bloat, royal gramma Gurgle, and damselfish Deb, led by Gill, a Moorish idol. That night, Nemo learns he is to be given to the dentist's young niece, Darla, whose rough treatment has killed most of her previous fish. Gill devises a risky escape plan: Nemo, who can fit inside the aquarium's filter tube, will jam the filter with a pebble, forcing the dentist to put the fish into plastic bags while he cleans the tank, giving them the opportunity to roll out the window and into the harbor. Nemo attempts the maneuver, but fails and is almost killed, leaving Gill guilt-ridden.
Marlin and Dory wake up unharmed, but the mask falls into a deep sea trench. Swimming after it, Marlin is chased by an anglerfish, while Dory reads the address and recites it repeatedly to commit it to memory. Dory and Marlin receive directions from a school of moonfish, but Marlin disregards their instructions to take what he believes is a safer route, leading them into a forest of jellyfish where they end up unconscious from all the jellyfish stings. Marlin and Dory wake up to find themselves on the East Australian Current with a group of friendly sea turtles including Crush and his son, Squirt. Marlin tells them about his quest, impressing them, and the story is relayed across the ocean, all the way to the dentist's office, where a pelican named Nigel tells the Tank Gang. Inspired by his father's bravery, Nemo makes another attempt to jam the filter and succeeds, and soon the aquarium's contents are covered in green algae.