Planet E9874 supports a developing civilisation known as the Tarl. The peaceful, technologically advanced Locoyuns are helping the Tarl develop rudimentary technology. What could be more innocent than that? When the Doctor, Leela and K9 arrive, they find the delicate balance in the relationship between the two cultures reaching an unexpected crisis point. The spears are flying and the threat of all-out war is in the air. The Doctor must use all his guile to tread a careful path with Tarl leader Ergu, while Leela and K9 discover an ancient power of unimaginable strength which threatens to tear the minds out of its victims.