In 1978, the first space probe to Saturn returns and the Seaview is sent to retrieve it. Despite their stringent decontamination procedures an alien life form survives and, once inside, begins to take over the ship.

By air Nelson and Crane pursue a mysterious submarine stealing nuclear missiles. Their craft is shot down and Crane is captured. Nelson, recovered by a Navy destroyer, is bent on destroying the sub, not realizing Crane is on board.