Two clownfish, Marlin and his wife Coral, admire the view from their new home within a sea anemone overlooking the drop off of a coral reef. Below them, their clutch of eggs lies hidden in a small hole. Excited to be first-time parents, they discuss names, Coral expressing her fondness for "Nemo." They flirt playfully with each other until Coral's attention is distracted by the appearance of a barracuda. Ignoring Marlin's order to hide, Coral moves to protect her eggs and the barracuda lunges. Marlin rushes in but the barracuda knocks him out with a flick of its tail, sending him back into the anemone. When he comes to that night, he discovers that Coral and the eggs are gone but manages to find a single surviving egg with a scratch on its right side. Vowing to protect it, he names the codling Nemo. Marlin raises Nemo in a secure anemone further into the reef. On the morning of his first day of school, an excited Nemo wakes his father, flapping his tiny right fin wildly. Marlin helps him prepare for the day, showing to be overprotective and doubtful of Nemo's ability to take care of himself. Marlin escorts Nemo to school -- along the way, Nemo asks Marlin how old sea turtles live to be; Marlin doesn't have an answer. Mr. Ray, the local teacher, takes the children on a field trip. When Marlin learns that they are going to the drop off, he swims after them in a panic. Upon arrival Nemo follows three of his peers, bored with Mr. Ray's lesson, to the very edge of the reef where they see a boat (misnaming it a "butt") anchored in the distance. They dare each other to swim out into open water to touch the butt as Marlin arrives and yells at Nemo for endangering himself. While talking to Mr. Ray, Marlin fails to notice Nemo swimming fiercely out to sea, stopping beneath the boat. In an act of defiance, Nemo touches the boat with his fin and starts to swim back before a diver suddenly appears behind him and traps him in a small bag. Marlin is prevented from swimming out after his son by another diver who takes a picture, disorienting him. Regaining his sight, Marlin swims after the divers as the boat departs the reef. The diver places Nemo in a cooler full of water and accidentally drops his mask into the water. Marlin swims after the boat but eventually loses the trail. He swims to the sea floor, begging passing schools of fish for help until he bumps into a regal tang named Dory. She claims to have seen the boat and leads a thankful Marlin in the direction it went but, as they swim along, she becomes lax and even tries to evade Marlin when she notices him behind her. When she confronts him, he questions her and is then told that she has short-term memory loss. Dumbfounded, Marlin turns to leave but is stopped by a great white shark who introduces himself as Bruce. He invites Marlin and Dory to a get-together he's having and, despite Marlin's objections, escorts them to his lair in a sunken submarine surrounded by live sea mines. They meet Bruce's fellow sharks, hammerhead Anchor and mako Chum, before beginning an assembly where they pledge to abstain from eating fish. When Marlin spots the diver's mask stuck on a shard of metal he discovers markings on the strap that might provide a clue to Nemo's whereabouts. Dory picks up the mask to see if the sharks can read but Marlin tries to take it back, engaging in a tug-of-war before the mask snaps into Dory's face, causing a nosebleed. The smell of the blood excites Bruce's inner carnivore and, though Anchor and Chum try to stage an intervention by holding him back, he mindlessly chases Marlin and Dory through the submarine, taking hold of the mask in his mouth in the process. During this time, Dory reveals that she can read human words before she and Marlin take refuge in a torpedo well. Dory releases the torpedo which wedges in Bruce's mouth, giving Marlin enough time to grab the mask and flee back into the well. Bruce throws the torpedo away and Anchor and Chum are able to regain his attention in time before the torpedo sets off one of the mines, causing a violent chain reaction of explosions from the others. Meanwhile, Nemo is placed into a new container revealed to be a fish tank in a dentist's office. There he meets a few of the original inhabitants including Bloat the puffer fish, Bubbles the yellow tang, Peach the ochre sea star, Gurgle the royal gramma, Jacques the French-accented Pacific cleaner shrimp, and Deb a black-tailed humbug who believes her reflection to be her twin sister, Flo. A pelican named Nigel perches himself on the window near the tank and greets Nemo after briefly discussing dental procedures with the other fish. He's shooed away by the dentist who shows Nemo a picture of his niece, Darla, whom Nemo has been promised to. The other fish cringe and call her a fish killer ('she wouldn't stop shaking the bag'). Afraid and wanting to go home, Nemo backs away and gets stuck in the suction tube of the filter. The other fish go to help him but Gill, a wise moorish idol with large scars on his right side, tells Nemo that he must escape himself, giving Nemo instructions and encouragement. Nemo manages to free himself and Peach relates his strength to his ocean origins, like Gill. Marlin and Dory awake in the aftermath of the explosions to find the submarine hanging precariously over the edge of a deep ravine. Their movements cause the submarine to suddenly tip forward and collide with the rock wall of the far side. During the chaos and blinded temporarily by the dust, Dory accidentally drops the mask into the darkness below. Marlin gives it up for lost but Dory cheerfully takes him into the depths, telling him to just keep swimming. After a moment, they come upon a mysterious and alluring light which, unfortunately, belongs to a hungry anglerfish. During the chase, Marlin finds the mask and distracts the anglerfish long enough so that Dory can read the address written on it. Marlin then leads the anglerfish towards the mask and traps it using the masks strap as a tether. Dory recites the address 'P. Sherman 42 Wallaby Way, Sydney' and is ecstatic to find that she can remember it. Back in the fish tank, Nemo is woken by Jacques who takes him to the tank's toy volcano (Mt. Wannahockaloogie) where the other fish perform an initiation to include Nemo in their club, giving him the nickname "Shark Bait." Gill then proposes a plan to escape the fish tank by blocking the filter which would cause the tank to become so dirty that the dentist will have to manually clean it and place the fish in baggies on the counter, allowing them to roll out the window to the street, cross it, and land in the harbor. Though the other fish are skeptical, Gill asserts that Nemo is small enough to fit into the filter and make it back through the tubing. Nemo agrees to do it. Marlin and Dory continue their journey and request directions to Sydney from a school of moonfish. They tell Marlin and Dory to take the East Australian Current (EAC), after an impressive display of their synchronized impressions. When they come to a rocky trench, Marlin resolves to swim over it to avoid the ominous enclosure. With the EAC in view, Dory becomes distracted and is stung by a tiny jellyfish. As Marlin tends to her, they are suddenly surrounded by a school of larger jellyfish. Finding that the tops don't sting, Marlin and Dory bounce on them as they race out of the school. Marlin makes it out first and, when Dory doesn't appear, goes back in to retrieve her. He finds her unconscious and scarred from the stinging and struggles to carry her out as he is repeatedly stung. He makes it but, stung and exhausted, blacks out as a large shadow descends on him. Nemo waits in the fish tank with Gill who is keeping an eye on the dentist for an opportune moment to jam the filter. Noticing Nemo looking at his scars, Gill explains that during his first escape attempt he landed on dental tools, though he was aiming for the toilet. He says that all drains lead to the ocean and that fish weren't meant to live in a tank. Peach cries out that the dentist is going for a bathroom break which cues Nemo to leap into the filter. Gill tosses him a pebble and gives instructions as Nemo wedges the fan, stopping the flow of water. However, as he is moving through the pipe back into the tank, the pebble unhinges and the fan turns on again, sucking Nemo backwards. The other fish hurry and send Nemo a toy kelp strand to grab onto and pull him out. Shaken, Peach asks Gill not to send Nemo in the filter again. Sullen and realizing his plan put Nemo in grave danger, Gill says they're done.