For ages past, the Union of Traken has lived in peace and harmony thanks to the power of the Source, controlled by generations of Keepers. But the current Keeper, his powers waning, senses some all-pervading evil about to invade his world. He summons the Fourth Doctor to his aid. To save Traken the Doctor fights the terrifying Melkur – only to find that this new enemy conceals an older and even deadlier foe – one the Doctor has encountered before ...