The Seaview is plumbing unexplored depths when it finds hundreds of metal cylinders on the ocean floor. One is brought aboard and found to contain a bald humanoid (Robert Duvall) in suspended animation, carrying a small side-arm. He revives and learns English in a few hours, introducing himself as Zar. He explains that his people inhabited the Earth millions of years ago and achieved a superior level of inteligence until their civilisation was destroyed by a natural catastrophy.
He asks Admiral Nelson (Richard Basehart) to revive the rest of his people in the other cylinders, but Nelson is suspicious and refuses. Crewmen begin to fall mysteriously ill until Zar reveals that he has been secretly infecting them with his own toxic blood. He threatens to kill the rest of the crew but Nelson cannot shoot or injure him without infecting the whole submarine, whereas Zar has a heat ray gun.
After a brief stand-off Nelson lures Zar into his office which he has boobytrapped by making it airtight and flooding it with a high oxygen explosive gas. Nelson escapes down the ventilation ducts, sealing them behind him as Zar attempts to blast his way out with his side arm. The heat ray ignites the gas and Zar is hygenically incinerated.