Troubled man Martin Clegg has always suffered from dreams which seem intensely real. In them, bizarrely, he's another person - not of this Earth! He's finally forced to confide in his fiancee, Elsie Barlow, and they consult Martin's scientifically inclined friend Tom Cavendish. He reveals, astonishingly, that Martin has a cosmic twin to whom he's mentally linked. Unsuspecting, they are about to become caught up in the strands of an incredible cosmic mystery that will, inexorably, be played out...