Aliens from the planet Centaur bring a capsule containing Commander Morton to Venus. They want to keep earth from invading their planet and kidnap Captain Crane in an attempt to bring the Seaview to their planet.

Admiral Nelson and Captain Crane are shocked to see a scientist shot to death during a live televised news conference viewed from the Nelson Institute. Especially shocking is that the killer appears to be Admiral Nelson. Both Nelson and Crane are eventually able to overcome efforts to stop them from boarding the Seaview. The submarine races to stop an earth-threatening project set in motion by the radical scientist, who may not really be dead after all.