A space probe returns from Earth, but it bears a strange stowaway: a mysterious form of alien life that can hide by reducing its mass and volume to nearly zero. Seaview recovers the probe; Nelson and Sharkey take the scientific package from it to a nearby aircraft carrier. While they're away, the alien form expands, and begins to take control of Seaview and her crew. Nothing seems to stop it, until Nelson realizes that it must have a brain, and that means it may have a key weakness that he can exploit. It's a race against time; Nelson must kill the monster before Seaview reaches the mainland!