The TARDIS accidentally lands in Mercy, a town in the American West around 1870. The TARDIS crew discovers the town’s doctor, Jex, is an alien who is being sought by the cyborg Gunslinger. The Doctor discovers Jex was a scientist who experimented on volunteers to create cyborgs to fight in a war on his home planet; the Gunslinger is seeking revenge for what was done to him. The Doctor faces a moral dilemma of whether he should offer Jex to the Gunslinger; he devises a plan to help Jex escape, but Jex commits suicide to save more innocent people from being harmed. The Doctor saves the Gunslinger from self-destruction and makes him the marshal of Mercy.