The Doctor, Amy and Rory encounter suspicious locals when they find themselves in the Wild West town of Mercy. The townsfolk are being terrorised by a mystery robotic gunslinger who kills anyone who tries to leave town. The cyborg wants the locals to hand over an ‘alien doctor’.
The town’s marshal, Isaac, reveals that another alien doctor has recently crash-landed nearby and that that he is hiding in the prison. The alien is known as Kahler-Jex and has been using his advanced knowledge to help the town battle an outbreak of cholera.
However, the Doctor learns Kahler-Jex is a war criminal who experimented on and weaponised his own people. The cyborg is one of his former projects and wants revenge for the horrors Kahler-Jex put his people through.
The Doctor decides to hand over Kahler-Jex, and comes to blows with Amy about his decision. When the Doctor attempts to hand him over, marshal Isaac jumps in the way and is shot by the cyborg – making the Doctor the town’s new marshal.
The townsfolk turn on the Doctor, who is now defending Kahler-Jex and is torn over whether the alien’s good work in the town has made amends for his terrible misdeeds.
The Doctor agrees to a duel with the cyborg to decide the matter, giving Kahler-Jex the chance to run to his spaceship. However, rather than escape, Kahler-Jex blows himself up to end the conflict before humankind is dragged into the gunslinger’s revenge campaign.
Once more, Amy and Rory decline to continue their adventure with the Doctor and return home, while the cyborg stays to protect Mercy forever more.