When a mass missile launching by a foreign power sets the U.S. on nuclear alert, the Seaview goes into a fail-safe maneuver, but is the launch really what it appears to be, and does this actually mean it's "doomsday?"

After returning from a mission in deep space, a lifelike robot is brought aboard the Seaview, but the crew soon discovers that the robot has been mysteriously reprogrammed to wreak havoc.