Doctor: Matt Smith (Eleventh Doctor)
Karen Gillan (Amy Pond)
Arthur Darvill (Rory Williams)
Ben Browder – Isaac
Adrian Scarborough – Kahler-Jex
Dominic Kemp – Kahler-Mas
Rob Cavazos – Walter
Joanne McQuinn – Sadie
Andrew Brooke – The Gunslinger
Garrick Hagon – Abraham
Byrd Wilkins – The Preacher
Sean Benedict – Dockery
Directed by Saul Metzstein
Written by Toby Whithouse
Produced by Marcus Wilson
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 7
Length 45 minutes
Originally broadcast 15 September 2012
The Doctor, Amy, and Rory arrive at the American Frontier town of Mercy. They find that no one has been able to leave or enter town due to actions of "The Gunslinger", who demands they turn over Kahler-Jex, an alien scientist that crashed to Earth about a decade earlier. The townspeople rescued him, and in exchange, he provided them with primitive electricity and the means to stop a cholera outbreak. The town's marshal, Isaac, is hesitant of turning over his friend to the Gunslinger, despite being near the end of their supplies.
The Doctor sneaks out to get the TARDIS but finds Jex's crashed ship and discovers he was one of several scientists that did inhumane experiments to create cyborgs on their war-torn homeworld. The Gunslinger arrives, and confirms the Doctor's theory that he is one of Jex's experiments. He has sought vengeance and killed all but Jex. Though he cannot harm innocents by his programming, the Gunslinger warns that Jex must be brought to him or he will take more drastic measures.
The Doctor returns to town, and angrily drags Jex to face the Gunslinger. Amy asserts that the Doctor has changed for the worse from months of travelling on his own. Jex pleads that he has atoned, but the cyborg prepares to kill him. Isaac pushes Jex out of the way and takes the lethal shot. As he dies, Isaac gives the Doctor his marshal badge and has him promise to protect Mercy. The Gunslinger departs, warning he will return for Jex at noon tomorrow.
Jex tells the Doctor he is remorseful for all those deaths his work has caused. The townsfolk demand the Doctor turn Jex over, but he reminds them that Isaac's death will be for naught if they do so, and has come up with a plan from learning about Jex' society. When the Gunslinger arrives the next day, residents wearing facial markings similar to Jex race through town, confusing the cyborg and allowing Jex to flee to his ship. However, instead of taking off, Jex tells the Doctor he is ready to face justice in the afterlife, and causes the ship to self-destruct.
The Gunslinger with no further purpose prepares to travel a distance to self-destruct, but the Doctor suggests a new resolve. As the Doctor, Amy, and Rory leave, the Gunslinger stands watch over Mercy as its new marshal.