The Doctor (Matt Smith), Amy (Karen Gillan), and Rory (Arthur Darvill) take a trip to the Old West where they come across a small town terrorized by a cyborg known as The Gunslinger (Andrew Brooke). The cyborg’s soul purpose is to wait for an opportunity to kill an alien doctor named Kahler Jex (Adrian Scarborough) who has taken refuge in the Sheriff’s (Ben Browder) office.
While attempting to keep the townspeople safe, and deviating from his original plan of evacuating the townspeople in the TARDIS, The Doctor finds Jex’s spaceship and uncovers the truth about the scientist and his connection to the cyborg who wants him dead. Jex was one of the scientist who experimented on countless soldiers to create the cyborg warriors to end a war on his home planet. And now the cyborgs have begun hunting the men who transformed them into monsters, seeking justice for what was done to them.
After spending too much time alone, and with his patience at an end, The Doctor’s decision is all too easy. The Doctor’s willingness to let Jex die to protect the town is only prevented by Amy’s plea for mercy and the Sheriff’s death which forces The Doctor into not only the role of the town’s savior but their new Marshall as well.
With no more time, The Doctor finds himself staring down The Gunslinger in the middle of Main Street at high noon. Of course he does have a plan (funny, I didn’t know The Doctor was a ¡Three Amigos! fan). However, as you might expect, the plan goes somewhat awry. But by the end of the episode the town of Mercy is safe, Jex’s debt paid, and the town has a new member to look after their safety. “A Town Called Mercy” is somewhat empty, and incredibly derivative, but for a mid-season standalone episode (something we’ve gotten far too little of the past couple of years) it works well-enough.