A nuclear submarine in the Pacific Ocean, under Captain Pete Matthews (Kenneth Tobey) detects a massive object with its sonar. The captain tries to outmaneuver it, but cannot, though he manages to free his vessel and it returns, damaged, to Pearl Harbor. They find tissue from a huge sea-creature jammed in the dive plates.

Professor Lesley Joyce (Faith Domergue) and John Carter (Ronald Curtis) arrive with a team of marine biologists from Harvard. They decide the tissue is from a giant octopus. The military, of course, does not believe them, but reports pile up of swimmers and craft being pulled under by a huge sea creature. The scientists think it came from the Mindanao Deep, driven out by hydrogen bomb testing and the loss of its food supply. The tissue tests positive for radioactivity. The recent disappearances of a Japanese fishing fleet and a Siberian sealing vessel may also be victims. Captain Matthews and the Navy doubt this and demand proof. Reports come in of an attack on a Canadian freighter. They question the handful of men who escaped in a raft, and when the first report is doubted, the rest clam up. Joyce convinces the man to repeat his testimony for the U.S. government. The government stops all traffic in the North Pacific. Carter flies out to sea to trace a missing ship, Matthews and Joyce (who are becoming intimate) follow up a report of five people missing off the coast of Oregon.

The local sheriff, Bill Nash (Harry Lauter) takes them to the site, and they find a giant suction cup imprint on the beach. They ask that Carter join them. The sheriff is attacked right in front of them. They escape and arrange for the Pacific coast to be mined, and then they leave for Navy Headquarters in San Francisco.

An electrified safety net is strung across the entrance to San Francisco Bay to protect the Golden Gate Bridge, which has also been electrified. Carter takes a helicopter along the shore, which he baits with dead sharks, hoping to lure the creature onto the land. Joyce demonstrates to the Press a jet-propelled atomic torpedo, which they think will kill it. The creature demolishes the underwater net and heads for San Francisco. The Golden Gate Bridge has been closed, but when Carter learns that an electric current has been left on, he rushes to shut it off. If this creature is like an octopus, it may be attracted to it. Matthews rescues Carter just before a section is brought down by a huge tentacle.

The populace panics and begins a mass exodus down the peninsula. The Navy evacuates the Embarcadero and the Ferry Building, which is battered to a ruin. When more people are killed, the Defense Department authorizes Matthews to attack it with the submarine and the torpedo. Carter joins him and Joyce remains behind.

Flamethrowers push the creature back, but when the torpedo is fired into it, the creature grabs the submarine. Using an aqualung, Matthews attaches explosive charges to it before being knocked out by a premature explosion. Carter swims out and shoots it in the eye, forcing it to release the submarine, and pulls Matthews to safety. As it swims away, the torpedo is detonated, destroying it. At a celebration in a restaurant, Matthews proposes, and Joyce accepts.

Producer Charles Schneer and Sam Kurtzman got the idea from a hydrogen bomb test in the Marshall Islands. The title was suggested by It Came from Outer Space. They were impressed by the Beast from 20,000 Fathoms and hired Ray Harryhausen, but funds were tight, and he was only authorized to make six tentacles. He called it a sixtopus. I figure what the hell, it’s a mutant. They were not allowed to shoot on the Golden Gate Bridge because the city thought the people might believe the bridge could be pulled down, so they put a camera in a bakery truck and drove back and forth across the bridge. The film was so successful that it led to Earth vs. the Flying Saucers the following year.

Some reviewers disliked the 45-minute wait until the creature showed up, but others thought the documentary style and clipped military feel gave the movie realism and suspense. Everyone loved the monster. A four-issue comic-book series called “It Came from Beneath the Sea…Again” was released by Tidal Wave Productions as part of a Ray Harryhausen Signature Series. Other titles were Wrath of the Titans, 20 Million Miles More, Sinbad Rogue of Mars, Jason and the Argonauts, Flying Saucers vs. the Earth, and Back to Mysterious Island.

Ray Harryhausen and Charles Schneer worked together until their last film Clash of the Titans, whereupon they both retired, Since San Francisco would not let them use the Golden Gate Bridge, Harryhausen built it as a miniature out of lead. Kenneth Tobey played a Navy Commander in this film, an Air Force Captain in The Thing from Another World, and an Army Colonel in The Beast from 20,000 Fathoms.

Faith Domergue was born 16 June 1924 or 1925 in New Orleans, of Irish, English, and Creole ancestry. She may have been a foundling as she was adopted by Adabelle Wemet when six weeks old. When she was 18 months, her adoptive mother married Leo Domergue, giving her his name. The family moved to California in 1928, where she attended Catholic School. She was a sophomore in High School when she signed with Warner Brothers and appeared in Blues in the Night. She attended a party on Howard Hughes’ yacht, and he bought her contract when she was 16 and she signed with RKO.

She was a sultry brunette type, compared with Jane Russell, and appeared with Robert Mitchum and Claude Rains. She was also Hughes’ girlfriend until she found out he was also seeing Ava Gardner, Rita Hayworth, and Lana Turner. Faith Domergue appeared in Cult of the Cobra, It Came from Beneath the Sea, This Island Earth, The Atomic Man, some British noir films, and The Sky Burns, plus TV shows like Cheyenne, 77 Sunset Strip, Have Gun—Will Travel, Bonanza, and Perry Mason. Later she appeared in Voyage to the Prehistoric Planet, some Italian thrillers, and the House of Seven Corpses. She married three times, to a bandleader and two directors. In the Hughes biopic the Aviator, she was played by Kelli Gardner.