When the TARDIS arrives on a jungle planet, the Doctor encounters two warring tribes, the Sevateem and the Tesh. The Sevateem worship a god called Xoanon and the Tesh are supposedly keeping Xoanon prisoner. But why do the Sevateem call the Doctor the Evil One? And what are the invisible creatures in the jungle? The Time Lord, with the help of a girl called Leela, is about to find out.

The Doctor, Sarah and Harry return to Earth. UNIT is investigating a series of attacks on North Sea oil rigs and has set up a temporary HQ in Scotland. The attacks are the work of a huge cyborg, the Skarasen, controlled by a group of aliens called Zygons whose spaceship lies at the bottom of Loch Ness. They plan to take over the Earth as a substitute for their own planet. They are using their shape-shifting abilities to take on the identities of locals...