The Doctor and Leela arrive on the planet Kaldor, where they find a society dependent on benign and obedient robots. But they have faced these robots before, on a huge Sandminer in the Kaldor desert, and know they are not always harmless servants. The only other people who know the truth are the three survivors from that Sandminer — and now they are being picked off one by one. The twisted genius behind that massacre is dead, but someone is developing a new, deadlier breed of robots. This time, unless the Doctor and Leela can stop them, they really will destroy the world. It is an adventure featuring the Fourth Doctor, as played by Tom Baker, and his companion Leela.