Kahler-Mas is pursued through the desert by the Gunslinger. Under a dark night sky, he is killed – but he is not the last one. There’s one more: the Doctor.
Ignoring the line of stones and lumps of wood in the desert, and a suggestive ‘Keep Out’ sign, the Doctor, Amy and Rory enter a town called Mercy. 81 residents with aggressive stares; electric lighting ten years before electric lighting; and strong tea (leaving the bag in): it’s all the Doctor could wish for. Until he tells everyone his name – and they immediately throw him out.
The Gunslinger appears in the desert, but when the Doctor tries to get back into Mercy, the residents point their guns at him. Fortunately, the town’s Marshal, Isaac, brings them back to their senses, and, as the Doctor rejoins the group, the Gunslinger disappears. The townsfolk hoped he was the ‘alien doctor’ that the Gunslinger is pursuing – but they know he isn’t really.
The real alien doctor, Kahler-Jex, is locked up in Isaac’s office. Jex explains that his ship crashed in the desert and the people of Mercy saved him from the wreckage. In return, he set up some rudimentary lighting and heating for the town and cured them of Cholera. This still doesn’t explain why the Gunslinger is after him – but Isaac says that the place is named Mercy for a reason. The Doctor agrees to use the TARDIS to help Jex and the townsfolk to escape… but first, they have to get past the Gunslinger.
Accompanied by Rory, Isaac dresses up as Kahler-Jex and is pursued by the Gunslinger, while the Doctor commandeers a male horse called Susan. However, the Doctor doesn’t stick to the plan; he follows the power cables supplying the town with electric to Kahler-Jex’s ship. But where’s the damage…?
The Gunslinger, meanwhile, corners Rory and Isaac, thinking the latter is Jex. Suddenly, the blaring alarm of Jex’s ship echoes across the desert.
Realising it’s the Doctor, Jex pulls a gun on Amy, intending to use her as a hostage in order to escape: the Gunslinger’s programming won’t let him harm innocents unless absolutely necessary (and Amy is coloured reassured!).
The Doctor enters the small ship and disables the alarm (and self-destruct), and looks at the personnel files of Dr. Kahler-Jex. He is the Military Science Advisor to the Experimental Cyborg Program Military Science Unit – responsible for the deaths of countless Kahler.
As Jex tries to make his escape, Isaac returns, and pulls a gun on him. He is forced to let Amy go.
The Doctor also has a gun aimed at him, as the Gunslinger waits outside the spacecraft. He pleads with the cyborg, a victim of Kahler-Jex, and reasons that the Gunslinger had shown the town mercy himself, not wanting innocents to get in the way of his personal vendetta. But now, they’ll be no more warning shots. He’ll kill the next person who steps over the line. “Make sure it’s Jex.”
The Doctor confronts Jex about his past; about how he experimented on members of his own race, merged them with metal, and programmed them to kill. But Isaac sees the good in him, and believes there must be a good reason. The Kahler had been at war for nine years, and Jex and his team were tasked with bringing peace – no matter what the cost. The cyborgs that he created ended the war in less than a week. Afterwards, the cyborgs were decommissioned, but one had got its circuitry damaged in battle. It went offline, and began hunting down those responsible – until just two were left.
Kahler-Jex and Kahler-Mas fled, their ships crashing to Earth, but pursued by the Gunslinger.
Weighed down by the responsibility of protecting Mercy, the Doctor throws Kahler-Jex out of town. Today, the Doctor honours the victims first: Jex’s, the Daleks’, the Master’s… And Amy realises that this is what happens when he doesn’t have anyone to travel with. She reasons with him. “We can’t be like him; we have to be better than him.”
But it’s too late. The Gunslinger appears, and Jex faces up to him, identifying him as Kahler-Tec, and begs for his life. The Gunslinger fires – but Jex is saved by Isaac, who is hit himself. Isaac hands the Marshal badge to the Doctor, and instructs him to save both Jex and his town, before dying in the Doctor’s arms. Appalled by another innocent’s life being lost at his hand, the Gunslinger retreats, but warns that it’s their last chance. He’ll return tomorrow at noon.
That night, the Doctor contemplates Jex’s fate. The townsfolk group up on him, however, scared by the Gunslinger’s threat, but he is able to talk them out of their rash decisions, telling them that it’s everything Isaac didn’t want. “Violence doesn’t end violence.”
The two doctors talk, and Jex shares that he hears their screams every time he closes his eyes. In a way, he’s a victim too. Helping Mercy is his way of atoning for his sins – but “justice doesn’t work like that. You don’t get to decide when and how your debt is repaid.” Jex fears his death, as the Kahler believe that your spirit has to climb a mountain, carrying the souls of all you have wronged. Imagine the weight Kahler-Jex will have to lift.
Noon. The Gunslingers enters town.
The Doctor, Rory, and other people of Mercy disguise themselves as Jex to trick the Gunslinger, while the real Kahler escapes to his ship. A little girl watches as the Gunslinger spares her fellow residents, and the Doctor confronts him, asking him to leave Mercy and saying that he’ll see Jex’s ship fly away imminently.
But a tannoy announces that Kahler-Jex, in his ship, is ending the war for both of them – and the spacecraft self-destructs.
The Gunslinger plans to self-destruct too, as he’s left with no purpose. So the Doctor offers him one…