John Peel

A visit to the Grand Canyon begins with disaster for the Doctor and Peri, when the TARDIS plunges into a ravine. The travellers' only hope is to descend to the valley on foot and then track the course of the Colorado river. Their perilous journey is fraught with danger, even before they encounter a band of 19th Century explorers led by Major John Wesley Powell. Joining the watery expedition, the Doctor and Peri begin to despair of ever seeing the TARDIS again. As food supplies dwindle, so do the group's options. This is a journey which some of them will never return.

Following an encounter with the Cybermen, the Doctor's life appears to hang in the balance - until, before the eyes of his companions Ben and Polly, his physical appearance is transformed. Disoriented after his regeneration, the Doctor takes the TARDIS to the Earth colony planet Vulcan. Ben and Polly are disturbed - the Doctor isn't the man he used to be.

The Doctor is also worried; the colonists have found the remains of two Daleks, which they plan to revive. Once reanimated, the Daleks claim that they are content to serve humanity. Can it really be true? Or do they have their own, more sinister plans.