The war between the Sontarans and the Rutans has been raging for millenia. 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 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 hundred. And strange objects have been observed orbiting the sun.

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