An unnamed planet, the distant future. Arriving in a place that seems oddly familiar, the Doctor finds the savage Sevateem tribe battling to free their captive god, Xoanon, from the Evil One and his Tesh servants. But can the whole situation really be the Doctor's fault?

The Sevateem launch another attack to try to free Xoanon as the Doctor tries to figure out his role in their past. But his only chance to win their trust is to face a deadly test.

The Doctor has broken through the barrier and found the Tesh. But he comes to an awful realisation about Xoanon.

The Doctor tries to fix the mad computer Xoanon. But it is determined to stop him – even if it must destroy the entire world!