The Doctor (Matt Smith) takes Amy (Karen Gillan) and Rory (Athur Darvill) to the planet Apaladucia for a holiday, but finds a plague raging there which kills the natives, who have two hearts, in a day. There are kindness facilities where the infected live in accelerated time-streams, living out their lives and watching their loved ones through a kind of window. (A little like Covid 19 lockdown.) Amy is separated from the Doctor and Rory and is stuck in an accelerated time-stream. Robots keep trying to treat her, but because she only has one heart, the treatment will kill her. The Doctor tells Amy to wait, and he will rescue her.
The Doctor is stuck in the TARDIS because, having two hearts, he could succumb to the plague, but gives Rory the sonic screwdriver and other tools so he can explore the complex. He finds Amy, 36 years older and embittered. The Doctor and Rory encourage older Amy to find younger Amy in another time-stream, but if she does, the older Amy will cease to exist. If they take the older Amy on board the TARDIS, they will not be able to find the younger one. Eventually, they are brought together and the younger one convinces the older one to change her mind by asking her to think of Rory. The Doctor brings the two Amys into the same time-stream. Rory takes the younger one into the TARDIS. The Doctor locks the older one out, who tells Rory to move on without her.
This was a low-budget doctor-lite episode, but it received positive reviews, mostly because of Karen Gillan’s performance. She played both Amys, using makeup and prosthetics and developing a different body-language for the aged version. It was nominated for a Hugo but lost to The Doctor’s Wife. The older Amy was modelled after Scathach N-‘unanaind, a red-headed Scottish warrior woman in Celtic mythology. In the Curse of the Black Spot episode, Amy wielded a cutlass; here it was a Samurai sword.
The cast included only the three main characters, plus one other, plus Imelda Staunton’s voice, and three uncredited actors playing robots. For cast-list brevity, it was beaten out by the William Hartnell episode The Edge of Destruction in 1963, which had only four characters, unless you count the TARDIS. Always thinking, in this episode the TARDIS brought the Doctor straight to the quarantine area, saving his life. As she had said, “I always took you where you needed to go.” The Doctor, however, came off a bit like a callous and manipulative liar.