"They've been fighting this war for longer than man has been walking upright, and they don't take prisoners."

The war between the Sontarans and the Rutans has been raging for millennia. Billions have died and whole star systems have been obliterated in the conflict. Now, finally, one side may have victory within its grasp.

The human colony world of Raghi is crucial to that victory. When the Fifth Doctor and Turlough arrive there, they find a seemingly stable society ruled by a strict caste system. But all is not as it seems. Members of the lower caste are being struck down by a mysterious illness. People are vanishing in their hundreds. And strange objects have been observed orbiting the sun.

Why is Raghi so important to the feuding alien empires? And how high a price will the galaxy pay if the conflict comes to an end?