Teen-aged Don Miller felt lucky to be one of the few aboard the atomic-powered submarine, the Triton, on its first official depth test run. Even when the ship began to falter and the diving planes jammed, neither he nor the rest of the crew realized they were fighting a losing battle against an unknown enemy. Quick repairs and a frantic attempt to surface brought renewed hope to the crew until they spotted strange-looking "bubble men" lashing through the water on creatures supposed to have been extinct millions of years before...