Description
A nuclear submarine on maneuvers in the Pacific Ocean, captained by Commander Pete Mathews (Kenneth Tobey), comes into contact with a massive sonar return. The commander attempts to outrun and outmaneuver the sonar object, but cannot. The boat is disabled but manages to free itself and return to Pearl Harbor. Tissue from a huge sea creature is discovered jammed in the submarine\'s dive planes.
The cephalopod attacking the Golden Gate Bridge
A team of marine biologists, Professor Lesley Joyce (Faith Domergue) and John Carter (Donald Curtis) of Harvard University, is called in; they identify the tissue as being a small part of a gigantic octopus. The military authorities scoff, but are finally persuaded after receiving reports of missing swimmers and ships at sea being pulled under by a large sea creature. Both scientists conclude that the animal is from the Mindanao Deep, having been forced from its natural habitat by hydrogen bomb testing in the area, which has made the giant octopus radioactive, driving off its natural food supply.
The scientists suggest the disappearances of a Japanese fishing fleet and a Siberian seal boat may be the work of the foraging giant. Both Mathews and the Navy representatives express doubt and demand further proof. Later, as Mathews assists Joyce and Carter, a report comes in of an attack on a Canadian freighter; several men escaped in a raft. The survivors are questioned by psychiatrists, and when the first sailor\'s description of a creature with giant tentacles is met with skepticism, the other sailors refuse to testify. Joyce is able to convince the first sailor to repeat his story for government officials, who then have the evidence they need. The U.S. government halts all sea traffic in the North Pacific without revealing the reason. Carter flies out to sea to trace a missing ship, while Mathews and Joyce follow up on a report of five missing people off the coast of Oregon.
The local sheriff, Bill Nash (Harry Lauter), takes Mathews and Joyce to the site of the attack, where they find a giant suction cup imprint in the beach sand. (At this point, the two have become physically intimate.) They then request that Carter join them. Nash is later attacked along the beach by the giant octopus, right in front of the two scientists. They escape, and together they hastily arrange for all Pacific coast waters to be mined before departing for San Francisco and the Navy\'s headquarters.
An electrified safety net is strung underwater across the entrance to San Francisco Bay to protect the Golden Gate Bridge, which has also been electrified. Carter takes a helicopter along the shoreline and baits the sea with dead sharks in an effort to lure the creature inland. Joyce demonstrates to reporters a special jet-propelled atomic torpedo, which they hope to fire at the giant octopus, while driving it back to the open sea before detonating the weapon. Later that day, the creature demolishes the underwater net, irritated by the electrical voltage, and heads toward San Francisco.
The navy orders the Golden Gate Bridge abandoned, but when Carter learns that the electric circuit there has been left on, he races to the bridge to shut it off. The giant creature, however, catches sight of the bridge and attacks it, the electrical voltage irritating it even more. Mathews is able to rescue Carter just before a bridge section is brought down by a giant tentacle.
The residents of San Francisco panic and begin a mass exodus down the peninsula. The navy struggles to evacuate the Embarcadero and the Ferry Building, which is battered by the creature\'s giant tentacles. When more people are attacked and killed, the Defense Department authorizes Mathews to take out the submarine and fire the torpedo; Carter joins Mathews while Joyce remains at the base.
Flamethrowers push the giant tentacles back into the sea, but when Mathews fires the jet torpedo into the giant creature, it grabs the submarine. Using an aqualung, Mathews swims up to the massive body and places explosive charges before being knocked out by the shockwaves from the premature explosion. Carter then swims out and shoots at one of its eyes, forcing the giant octopus to release the submarine; he then pulls Mathews to safety. Back at the base, as the creature turns toward the open sea, the torpedo is detonated, completely destroying the giant cephalopod. The trio later celebrate the victory at a restaurant, where Mathews makes an impromptu proposal, and Joyce accepts.
After an encounter at sea with an unknown underwater creature, a naval commander works with two scientists to identify it. The creature they are dealing with is a giant, radioactive octopus that has left its normal feeding grounds in search of new sources of replenishment. As the creature attacks San Francisco, the Navy tries to trap it at the Golden Gate Bridge but it manages to enter the Bay area leading to a final confrontation with a submarine.
