Three thousand years ago, the voracious Lom attempted to invade and destroy the planet Sorsha, but the Sorshans sealed their world behind an impenetrable barrier and released a virus which destroyed the Lom forever. The barrier has finally lifted itself, and the Seventh Doctor takes Ace to pay his respects to the dead Sorshans, only to find a contingent of Earth marines sent to salvage Lom technology from the planet. Their linguist, Skrane, manages to translate enough of the Lom language to enable Lieutenant Frethil to activate a machine called "the Grif" - which begins to generate a new army of Lom warriors. The Lom slaughter the marines and place their bodies in the Grief, providing it with enough raw genetic material to create a new army. Soon the Lom will be released upon the galaxy again, and the sacrifice of the Sorshans will be rendered meaningless. Skrane, blaming himself, sends the Doctor and Ace to safety while he repairs and reactivates the barrier controls. The Doctor and Ace escape just as the Lom break into the control centre and kill Skrane - but not before he reactivates the shield and virus dispersal units, ridding the Universe of the Lom once more.