Corpse Marker

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.

4 out of 6 (66.7%) raters say this story requires a previous story.