Word comes through that Dr Liscomb has been kidnapped by a foreign agent posing as a toy importer. Liscomb has cracked the secret of the X factor, a new ingredient and everyone wants it.

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.