Marlin is a clownfish who lives in an anemone in the Great Barrier Reef. His wife, Coral, and most of their eggs are killed in a barracuda attack. Only one damaged egg remains, which Marlin names Nemo. Years later, Marlin is overprotective of Nemo. On Nemo’s first day of school, Marlin embarrasses Nemo, and the two fight.
While Marlin is talking to Nemo’s teacher, Nemo defiantly approaches a nearby speedboat, where he is captured by a pair of scuba divers. Marlin pursues the boat in vain and meets Dory, a blue tang who suffers from acute short-term memory loss, who offers her help. The two encounter three sharks who’ve sworn to abstain from eating fish. Marlin discovers a diver’s mask that fell from the boat; he accidentally hits Dory with it, giving her a nosebleed. The scent sends one of the sharks into a feeding frenzy, but they flee after accidentally setting off a ring of old naval mines, which knock Marlin and Dory unconscious.
Nemo is placed in an aquarium in the office of dentist Philip Sherman in Sydney, Australia. 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. Nemo learns he is to be given to Sherman’s niece, Darla, who has killed 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 Sherman 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.
Marlin and Dory wake up unharmed, but the mask falls into a deep trench. They descend after it and encounter an anglerfish which chases them. Dory memorizes the address written on the goggles, and they escape. Dory and Marlin receive directions from a school of moonfish, but Marlin disregards them to take what he believes is a safer route. They stumble into a forest of jellyfish, the stings of which knock them unconscious. They awaken in the East Australian Current with a group of sea turtles including Crush and his son, Squirt. Marlin tells them about his quest, and the story is relayed across the ocean to Sydney where a pelican, 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 is covered in green algae.