Taking place six months after the first film, Dory suddenly recalls her childhood memories. Remembering something about "the jewel of Monterey, California", accompanied by Nemo and Marlin, she sets out to find her family. She arrives at the Monterey Marine Life Institute, where she meets Bailey, a white beluga whale; Destiny, a whale shark; and Hank the octopus, who becomes her guide.
Two ocellaris clownfish, Marlin and his wife Coral, admire their new home in the Great Barrier Reef and their clutch of eggs when a barracuda attacks, knocking Marlin unconscious. He wakes up to find that Coral and all but one of the eggs have disappeared. Marlin names this last egg Nemo, a name that Coral liked.
Jonas Taylor est un ancien capitaine de la Marine et un plongeur spécialisé dans les eaux profondes. Il est recruté pour plonger dans l'océan Pacifique, pour sauver une équipe de scientifiques coincée dans l'épave d'un submersible attaqué et endommagé par un requin préhistorique de vingt mètres de long, connu sous le nom de Mégalodon. Engagé par un océanographe chinois, Taylor devra surmonter ses peurs et affronter une deuxième fois le prédateur qu'il a auparavant rencontré lors d'une expédition pour sauver des hommes et des femmes piégés dans les profondeurs de l'océan.
While a group of teenagers are having an evening beach party on Amity Island, Chrissie Watkins (Susan Backlinie) goes skinny dipping in the ocean. Something violently grabs and pulls her underwater. In the morning, Deputy Hendricks (Jeffrey Kramer) finds her remains on the beach. Upon hearing from the medical examiner that the cause of death was a possible shark attack, Police Chief Martin Brody (Roy Scheider) proceeds to close the beaches, but Mayor Larry Vaughn (Murray Hamilton) overrules him, fearing news of a shark attack will ruin the summer tourist season, the town's primary revenue source. He rationalizes that there have been no prior shark problems in the area, and theorizes the girl was killed in a boating accident. The coroner concurs and Brody reluctantly accepts their conclusion.
In Reef City, an underachieving bluestreak cleaner wrasse named Oscar fantasizes about being rich and famous while making his way to work as a tongue scrubber at the local Whale Wash, a job in which he is following in his father's footsteps. Soon after arriving he is called to the office of his boss, a pufferfish named Sykes, to discuss the fact that he owes "five thousand clams" and has to pay it back by the next day. After explaining this to his angelfish best friend Angie, she offers him a chance to pay back the money by pawning a pink pearl that was a gift from her grandmother. Oscar brings the money to the race track to meet Sykes, but becomes distracted by his dreams of grandeur. Upon hearing that the race is rigged, he places it all on a long-shot bet by the name of "Lucky Day". Such a million dollar bet is noticed nearby by a beautiful lionfish named Lola, who flagrantly seduces an excited Oscar, but Oscar is disappointed when she leaves upon learning that he is a whale washer. Sykes is furious that Oscar bet the money but nonetheless agrees to see how the race turns out. Moments before their betted "horse" crosses the finish line, he trips and falls on the line.
Prior to a new hotel opening on Amity Island, a huge great white shark ambushes two divers who are photographing the wreck of the Orca, the late Quint's boat. A few days later, the shark devours a water skier. The speedboat driver defends herself by dumping gasoline on the shark (accidentally spilling some on herself) and shooting it with a flare gun, causing the boat to explode. The shark escapes but is severely scarred.
At Aquatica, a remote former submarine refueling facility converted into a laboratory, a team of scientists is searching for a cure for Alzheimer's disease. Fluids from the brain tissue of three Mako sharks are being harvested as a cure for Alzheimer's. Unknown to the other scientists, Dr. Susan McAlester (Saffron Burrows) has violated the code of ethics and has genetically engineered the sharks to increase their brain size, but this has the side effect of making the sharks smarter and more dangerous.
At his son's wedding party, Edward Bloom (Albert Finney) tells the same tall tale he's told many times over the years: on the day Will (Billy Crudup) was born, he was out catching an enormous uncatchable fish, using his wedding ring as bait. Will is annoyed, explaining to his wife Joséphine (Marion Cotillard) that because his father lives in a fantasy world and has never told the straight truth about anything, he felt unable to trust him. He is troubled to think that he might have a similarly difficult relationship with his future children. Will's relationship with his father becomes so strained that they do not talk for three years. But when his father's health starts to fail from cancer, Will and the now pregnant Joséphine return to his hometown in Alabama to visit. On the plane, Will recalls his father's tale of how he braved a swamp as a child after he was dared by a few other children. He meets a witch (Helena Bonham Carter). She shows Don Price and another boy how they were going to die. They run away, frightened. When the witch shows Edward his death in her glass eye, he accepts it without fear. With this knowledge, Edward knew there were no odds he could not face.
A great white shark follows a team of water skiers. The driver stalls the ski boat but manages to restart it before the shark can approach. The shark follows the water skiers into the SeaWorld park and throws the gate off its rails while it is closing. Meanwhile, Florida announces the opening of SeaWorld's new glass covered underwater tunnels that allow close up views of the park's animals.
London-based gangster George Thomason and his right-hand man, Ken Pile, an animal lover with a bad stutter, plan a jewel heist. They bring in two Americans to help: con artist Wanda Gershwitz and "weapons man" Otto West, a loud-mouthed Anglophobe who fancies himself an intellectual and hates being (correctly) called stupid. Wanda and Otto are lovers, but they hide this from George and Ken, pretending to be brother and sister, so Wanda can work her charms on them. The robbery goes well, and the thieves get away with a large sum in diamonds. However, they are briefly spotted during their getaway by Mrs. Coady, an elderly lady walking her three dogs. The group then hide the loot in a safe in an old workshop. Soon after, Wanda and Otto betray George to the police and he is arrested. They return to collect the loot, with Wanda planning to double-cross Otto as well, only to find that George has moved it. Wanda decides to seduce George's barrister, Archie Leach, so that he can persuade George to plead guilty and give up the location of the diamonds. Unhappily married, Leach is brow-beaten by his vindictive wife, Wendy, and spoiled daughter, Portia, and he quickly comes under the spell of Wanda's charms.
Daniel Kintner (Daniel Travis) and Susan Watkins (Blanchard Ryan) are a couple frustrated that their hard-working lives do not allow them to spend much time together. They decide to head out on a scuba-diving vacation to help improve their relationship. On their second day, they join a group scuba dive. A head count is taken, and the passenger total is recorded as 20. Daniel and Susan decide to separate briefly from the group while underwater. Half an hour later, the group returns to the boat. Going by the tally sheet, the total comes to 20, though in reality, it is 18. Daniel and Susan are still underwater and have not yet realized that the others have returned. The boat leaves the site. Not long after, Daniel and Susan return to the surface and look for it. They believe the group will return to recover them in reasonable time.
On Amity Island, Chief Martin Brody (Roy Scheider), the hero of two previous shark attacks, has died from a heart attack. His wife, Ellen (Lorraine Gary), attributes it to the fear of sharks. She now lives with Brody's younger and more obedient son Sean (Mitchell Anderson) and his fiancée Tiffany (Mary Smith). Sean works as a police deputy and is sent to clear a log from a buoy a few days before Christmas. A massive 25-foot great white shark attacks him, severing his arm, then pulling him underwater and killing him.