The Doctor (Matt Smith), Amy (Karen Gillan), and Rory (Anthur Darvill) arrive in the TARDIS in the town of Mercy in the Old West. No-one can leave or enter the town because of a cyborg gunslinger (Andrew Brooke) who demands Kahler-Jex (Adrian Scarborough), an alien scientist who crash-landed years before and has given the town electricity and ended a cholera outbreak. The town Marshall, Isaac (Ben Browder), does not want to turn over their benefactor.

The Doctor gets to his TARDIS but finds Jex’s ship and discovers he performed inhumane experiments on a war-torn planet to create cyborg soldiers, including the Gunslinger after him now, who cannot harm anyone else, but can still make life horrible for the town, which is running out of supplies.

The Doctor returns and confronts Jex and drags him out to face the Gunslinger. Amy thinks the Doctor has been alone too long and is in a dark place. Lex pleads that he has changed, but the cyborg prepares to kill him. Marshall Isaac takes the bullet for him and gives the Doctor his badge before he dies, making him promise to protect Mercy. The Gunslinger warns he will return for Jex tomorrow noon.

Jex is remorseful. The town is willing to abandon him, but the Doctor says that will make Isaac’s sacrifice meaningless. He has learned about Jex’s society and has a plan. When the Gunslinger arrives, townspeople made to look like Jex run everywhere, confusing the cyborg, allowing Jex to flee to his ship, but Jex causes it to self destruct, committing suicide to atone for his crimes. The Gunslinger, with no purpose anymore, wishes to self-destruct, but the Doctor makes him Marshall to protect the town forever.

The episode was filmed at Fort Bravo and Mini-Hollywood in Almeria, Spain, where the likes of a Fistful of Dollars and The Good, the Bad, and the Ugly were filmed. The Doctor Who crew loved making a Western, for the first time since 1966, with The Gunfighters. Murray Gold wrote Western-style music for it. The cyborg was made to resemble Frankenstein’s Monster, ironically, to make it seem more sympathetic. It took three and a half hours to do his makeup. Ben Browder, from Farscape and Stargate SG-1, makes a great Marshall. Scott Bakula had been offered the role. Adrian Scarborough (Jex) known for The Madness of King George and Upstairs, Downstairs, stole the show. Like most Westerns, the episode was a morality tale. It is 97 years since the Doctor “died” at Lake Silencio. The Doctor speaks Horse, incidentally, as well as Cat and Baby.