A clownfish named Marlin and his wife, Coral, experience a blissful life within their home, an anemone nestled in the stunning Great Barrier Reef. They are on the brink of parenthood, eagerly awaiting the hatching of their multitude of eggs. However, disaster strikes when a menacing barracuda invades their peaceful abode, knocking Marlin unconscious. When he regains consciousness, he faces the heartbreaking realization that Coral and nearly all of their eggs have fallen prey to the predator, leaving only one fragile egg behind. Determined to protect his sole surviving child, Marlin is resolute in his mission to keep Nemo safe.
As time passes, Marlin's concern evolves into overprotectiveness. On the day Nemo is set to embark on his first day of school, the two engage in a fiery argument, emphasizing the clash of their contrasting views on parental care. In a show of rebellion, Nemo swims towards a nearby speedboat, only to be ambushed and captured by scuba divers. Frantic and desperate, Marlin launches into a wild chase, yet his efforts prove futile. In his quest, he encounters Dory, a blue tang fish afflicted with short-term memory loss, who kindly offers her assistance. Together, they face thrilling adventures, including a meeting with Bruce, Anchor, and Chum, a trio of sharks that have promised to abstain from eating fish. An incident occurs when Marlin inadvertently injures Dory with a diver's mask, triggering Bruce’s hidden instincts. The chaos culminates in an explosion of naval mines, but luckily, the group manages to escape unscathed.
Meanwhile, Nemo finds himself imprisoned in a fish tank at the office of dentist Philip Sherman in Sydney. There, he encounters the "Tank Gang," a group of fish led by Gill, a scarred Moorish idol keen on returning to the ocean. Sherman plans to gift Nemo to his troublesome niece, Darla, notorious for having harmed her previous pet fish. Bonding over their shared desire for freedom, Gill formulates an escape plan that involves Nemo using a pebble to block the tank's filter tube, prompting Sherman to bag the fish while cleaning the aquarium. As Nemo attempts this daring feat, his failure nearly leads to his demise, leaving Gill riddled with guilt.
The diver’s mask, which is the key to finding Nemo, falls into a deep trench. Marlin and Dory venture into the abyss searching for it but are faced with a terrifying anglerfish. Utilizing their wits, they trap the creature, and Dory memorizes the address on the mask, marking a pivotal moment in their journey. Enlisting the aid of a school of moonfish, they navigate to the East Australian Current. Along the way, they become ensnared in a jellyfish forest, suffering painful stings, and are knocked unconscious, only to awaken amidst a bustling current with a group of friendly sea turtles, including Crush and his adventurous son, Squirt. The tale of Marlin's quest spreads through the ocean, eventually reaching Nigel, a pelican who keeps in touch with the Tank Gang. Inspired by Marlin’s determination, Nemo tries his plan again and successfully blocks the filter, resulting in a tank covered with algae.
After their exciting adventures in the current, Marlin and Dory find themselves lost only to be swallowed by a blue whale. However, the whale helps them on their journey by expelling them into the waters of Sydney Harbour. It’s here that Nigel spots them and flies them to the office of Sherman, where he discovers that Nemo is playing dead to avoid Darla’s clutches. Misunderstanding the situation, Marlin is heartbroken, believing Nemo is lost forever. But with Gill's guidance, Nemo escapes through the sink drain leading to the ocean.
In the aftermath of their harrowing experiences, a changed Marlin bids a sorrowful farewell to Dory and heads home, only to find unexpected reunification with Nemo. Nevertheless, a fishing trawler captures Dory and a school of groupers in its net. It’s a moment of teamwork, as father and son strategize, instructing the captured fish to swim downward. Their collaboration ultimately frees everyone from the trawler's grip.
Time passes, and now a more confident and outgoing Marlin accompanies Nemo on his first day at school. Dory joins him as they wave goodbye to Nemo, filled with pride for their little one’s growth. Meanwhile, the Tank Gang, after their new plans unfold following the dentist's filter malfunction, finally escapes into the open waters of Sydney Harbour, still confined in their plastic bags but ready for bold new adventures ahead.
