An alien spacecraft crashlands on the Moon, its passengers refugees from a dying world. As their destination is Earth, they generously offer to take one person with them when they depart. The scheming Commissioner Simmonds takes steps to ensure that he is the lucky individual chosen, even if it means destroying Alpha.

Ship's engineer Patrick Osgood believes he can obtain titanium, to make a new heart for his sick wife Michelle, from the catacombs of the moon, using dynamite. The explosion which follows gives him a vision of the future in which the ship is pursued by a fireball and this comes true. Though Helena is making a new heart for Michelle, Osgood becomes impatient and drags his wife back into the catacombs, giving Koenig a double task - to avoid the fireball and save the Osgoods.