During a routine training missile, a nuclear powered submarine comes under attack.  After repeated attempts, radiation levels begin to rise on the ship until finally it break free.  They surface to check for damage and discover that piece of rubbery material is lodged in the stern diving planes of the submarine.

They head back to Pearl Harbor for repairs and an investigation begins which brings in two top scientists to attempt to discover what the material was that was found lodged in the diving planes of the submarine.  Pete Matthews, who was commander on the submarine is on hand to help the scientists in any way they can.  The scientists, Professor John Carter and Lesley Joyce seem to have their hands full but not enough that Matthews hasn't noticed that Lesley is a more than beautiful woman.  At first he hopes for a romantic interest but begins to feel that their is more than a professional attachment to Professor Carter.

When all tests are completed, they find the material to be a piece of a gigantic octopus but the Navy isn't ready to accept their results.  In spite of all of the evidence, including recent sinkings of a Japanese fleet but the Navy is reluctant to make a committment to the discovery.  Lesley and Joyce are released by the Navy to return to their civilian duties.

That night, a shipping vessel is attacked as a huge slimy tentacle rises out of the sea.  The ship doesn't stand a chance as the creature grapples with it, as the men scramble for their lives on board before the creature pulls the vessel under the waters.

The Navy gives Lesley and John Carter a farewell dinner as Pete Matthews tells them all of their passes to return home will be ready in the morning.  Pete takes Carter's advice on making his own luck and tells Dr. Joyce of his romantic interest in her. Their dinner is interupted when it is announced that their release is being recinded because of the sinking of the shipping vessel

Amazingly, survivors are found, but after one is interviewed and tells his remarkable story, they all become reluctant to repeat what they have seen for fear of being committed to an institution.  Dr. Joyce decides to use her womanly wiles to entice the man to repeat his story.  She presses a button on the inter-office intrcom and the Navy hears the whole unbelievable story.

Now the Navy goes into action cosing the North Pacific, planting mines and surveying the ocean waters in an attempt to find and destroy the creature. They request that any unusual occurances along the coast be reported immediately. Poor fishing is reported off the Northwest coast and a family vehicle is reported abandoned and overturned on the beach. Dr. Joyce and Pete Matthews go to a Harpers Cove to investigate and discover huge round shapes on the beach.  Like the Navy, the local authority laughs at the idea of a giant "sea serpent" menacing the beaches. Dr. Carter is told to report to the beach. He arrives in time for the creature to make an appearence killing the town deputy but giving the Navy the unreputable visual proof they need.

The creature heads towards the San Francisco Bay Area.  Using round the clock surveillance, the Navy uses all means at hand to locate and destroy the creature knowing that the beast has only one vital spot, it's brain.  It must be destroyed in one quick action.  A new war head patterned after a harpoon is deviced at will spear the beast and then it is exploded electronically.  Once again the monster surfaces, attaches itself to the Golden Gate bridge. Dr. Carter, realizing that an electornic fence that was attached to the bridge hasn't been activated attempts to turn it on as the Golden Gate bridge is destroyed under the heavy mass of it's body and cruching tentcles. With the bridge destroyed, the monster turns it attention towards the Bay area as it crushes cars, buildings and people under the slimy mass of it's monstrous tentacles. Flame throwers divert the attack.  

In an effort to finally stop the beast, the Navy submarine once again confronts the creature and launches the new torpedo into the creature.  Because the ship is to close to the beast they must launch the torpedo and make a run for it before detonating the bomb.  The torpedo is fired but the submarine becomes trapped by the creature. Pete Matthews put on scuba gear and, armed with an electronic gun, tries to free the submarine from the monster's grip. In the process he is knocked unconscious.  Dr. Carter then suits up and tries to rescue Matthews. The bomb is exploded and the creature is finally destroyed.  Leavind Matthews and Carter still in the water.  The two are rescued and the sea is once again safe.

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IT CAME FROM BENEATH THE SEA STORE