En 1944, alors que la guerre du Pacifique fait rage, deux pilotes s'écrasent sur une île inconnue. L'un est Américain, l'autre est Japonais. Alors qu'ils s'affrontent sur place, ils sont brusquement interrompus par une gigantesque créature.
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.
Off the coast of Alaska, oceanographer Emma MacNeil (Deborah Gibson) is studying the migration patterns of whales aboard an experimental submarine she took without permission from her employer. Meanwhile, a military helicopter drops experimental sonar transmitters into the water, causing a pod of whales to go out of control and start ramming a nearby glacier. In the chaos, the helicopter crashes into the glacier, and the combined damage breaks the glacier open, thawing two hibernating, prehistoric creatures. MacNeil narrowly avoids destruction as, unknown to her, a giant shark and octopus are freed. Some time later, a drilling platform off the coast of Japan is attacked by the octopus, which has tentacles large enough to wrap around the entire structure. After returning to Point Dume, California, MacNeil investigates the corpse of a beached whale covered with many bloody wounds. Her employer Dick Richie (Mark Hengst) believes them to be from a tanker propeller, but MacNeil insists they appear to be from a creature. Later, she extracts what appears to be a shark’s tooth from one of the wounds. Elsewhere, the huge shark leaps tens of thousands of feet into the air from the ocean and attacks a commercial aircraft, forcing it to crash into the water.
The film takes place in Tokyo in 2003, where two reporters, Hana Izumi and Hyuga Kanata, have arrived to document unusual seismic disturbances that have been picked up around Tokyo. The disturbances are originally thought to be aftershocks left over from a supposed massive earthquake on the Kanto Fault that occurred two years previously, causing hundreds of thousands of casualties and several hundred billion dollars in damage(possibly referencing the real-life Fukuoka earthquake or the Miyagi earthquake), although careful analysis of the evidence suggests otherwise.
When young history professor Russ is called upon by his sister to execute their late mother's estate, he is reunited with boyhood friend Mike, and with his father, the charismatic leader of a New Age cult. While exploring his memories, Russ wanders into a warehouse where hundreds of names are listed on the walls. As he sleeps that night, he dreams of a stone cudgel and awakens to find a cudgel in his motel room; the town drunk warns him that it is an instrument of sacrifice. A young liquor store clerk enlists him to help find her brother, whom she believes has been taken by the cult. Russ's aunt, who has been living in an asylum, tells him that his mother left a message hidden in her house.
During the Cuban Missile Crisis, a Russian sub is sunk while en route to Havana. As the sub goes down, the hold is breached and barrels full of some mysterious substance tumble out. Years later, an American nuclear submarine is transporting a captured terrorist to the States. The terrorist's henchmen, however, are planning to hijack the sub and rescue their leader. Meanwhile, a large mutated octopus is approaching the sub at high speed.
Amidst a storm, a boat is being piloted across the South China Sea by John Finnegan (Treat Williams). With him are his crewmen Joey "Tooch" Pantucci (Kevin J. O'Connor) and Joey's girlfriend Leila (Una Damon). Finnegan has been hired to transport a group of mercenaries, led by a man named Hanover (Wes Studi), to an undisclosed location in the middle of the ocean.
The film begins when a couple disappear while having a romantic night out on a yacht near the Pacific Northwest resort community, Grave's Point. The next day, local fisherman Whip Dalton (William Petersen) finds the empty lifeboat from the missing yacht and finds a large claw stuck into the boat. Whip sends the claw to a university to be analyzed and it ends up in the hands of marine biologist Dr. Herbert Talley (Ronald Guttman), who comes to Grave’s Point claiming it is from the tentacle of a Giant squid. The island harbor master Schuyler Graves (Charles Martin Smith) tries to hire Whip to kill the squid, but he refuses, so Graves sends out Lucas Coven (Larry Drake) instead. Coven succeeds in slaying the squid and the carcass is promptly sold by Graves to Sea Land Texas owner Osborne Manning (Denis Arndt) then unknown to the fishermen the sonar that isn't being watched detects another squid. It turns out the squid is much bigger than the one they killed and instead of following the boat or attacking, it resubmerges.
The film takes place along a stretch of coastline somewhere in Florida in the United States, where a local tourist spot has become plagued by a mysterious marine creature. Little do they know, the monster is the product of a secret military experiment — a genetic hybrid mutated from a common octopus and the prehistoric super-predator Dunkleosteus. Unfortunately, the creature has broken loose, and is now feeding on swimmers and tourists swimming or sailing along the coast. Also, the monster is only an infant, and will continue to grow if it is left to hunt much longer.
Ocean Beach, a seaside tourist resort, has come under attack by a giant octopus, which captures and devours human swimmers and boaters, picking the skeletons clean of flesh and bone marrow. Marine expert Will Gleason (Bo Hopkins) and crusading newspaper reporter Ned Turner (John Huston) suspect the construction of an underwater tunnel by the Trojan company, owned by Mr. Whitehead (Henry Fonda). Trojan has been using radio signals that have been "above regulated levels", somehow maddening the octopus and causing it to attack human beings. In the end, the octopus is killed by Gleason's pair of killer whales.
The Helios 7 space probe is sent on a mission to study the planet Jupiter. While on its outward journey to the gas giant, the probe is overtaken by the Space Amoeba, an amorphous parasitic extraterrestrial. The probe returns to Earth and crashes into the South Pacific, where the Amoeba leaves the device and inhabits the body of a cuttlefish, causing it to mutate into what is called "Gezora." The tentacled kaiju begins attacking ships and islands in the area.
As the film opens, a small boat is seen chugging through stormy seas. A giant octopus appears from the ocean and seems bent on killing the sole crew member on deck. Suddenly, the octopus releases the man and retracts its tentacles from the boat. Relieved, the sailor peers out the porthole to see Gaira, a large green man-like creature, fighting the octopus. After easily defeating it, Gaira turns his attention to the boat and sinks it.
The prologue is set in Nazi Germany during the final days of World War II. A Kriegsmarine Officer, flanked by three Commandos, barges into the laboratory of a Dr. Riesendorf with orders to seize the immortal heart of the Frankenstein Monster, on which Riesendorf is busy experimenting. The heart is summarily transported by U-Boat to be passed off to their Japanese allies via the Atlantic. In the Indian Ocean, off the Maldives, the U-Boat meets up with a Japanese Imperial Navy submarine to make the exchange. They are sighted by an Allied Forces scout plane and bombed, but not before the Kriegsmarine pass the heart (contained in a locked chest) to the Japanese, who take it back to Hiroshima for further experimentation. But just as the experiments are about to begin, Hiroshima is bombed by the Allied Forces, and the heart and the experiments vanish in the atomic fireball.
Mr. Tako, head of Pacific Pharmaceuticals, is frustrated with the television shows his company is sponsoring and wants something to boost his ratings. When a doctor tells Tako about a giant monster he discovered on the small Faro Island, Tako believes that it would be a brilliant idea to use the monster to gain publicity. Tako immediately sends two men, Sakurai and Kinsaburo, to find and bring back the monster. Meanwhile, the American submarine Seahawk gets caught in an iceberg. The iceberg collapses, unleashing Godzilla (who, in the Japanese version, had been trapped within since 1955), who then destroys the submarine and a nearby military base.