Arriving on a jungle planet, the Doctor and Leela soon find themselves hunted by a hideous alien life form that appears to be some kind of robot with a taste for human flesh. But who has created it?A band of humans arrives on the planet in search of something deadly and illegal -- the robot? Or the things that created it?Assuming the Doctor to be involved in the monstrous crimes against nature on the planet, the humans make life difficult for the TARDIS crew -- but who is waiting and watching in the shadows -- and for whose benefit is the arena of peril set up?