Zoe, now in her late fifties, is approached by a young woman, Ali, who's positive she's met her — a much younger version of her — before. She's aware that her memories have been tampered with, and accepts Ali's offer of help to try and reconstruct the events of their previous encounter. Fortunately Ali has the necessary equipment with her, so they can get started straight away...
Ali, it transpires, was working at the Whitaker Institute in Australia when the TARDIS showed up, with the Doctor, Jamie and the younger Zoe. Exploring the facility, they discovered computer records detailing patients with terminal diseases — who, apparently, were nevertheless discharged. Then they ran into another intruder, who was looking into stories of strange goings-on, involving the word 'Achromatics'. When he set an alarm off, guards arrived and in the confusion Zoe got separated from the others. She found herself in a room with unknown alien lifeforms floating in vats: the Achromatics.
The Achromatics, it turned out, had unusual powers. But these weren't supposed to include roaming the Institute and killing people...
Tropes
- Absence of Evidence: The Doctor realises that the head of the facility genuinely didn't know about the reporter's death because he didn't try and make a story to explain it.
- Amnesia Missed a Spot: Ali tries to invoke this in reverse by claiming that Zoe just doesn't remember that she was present.
- Call-Back: Ali's employers have listened to Zoe's counselling sessions from Fear of the Daleks.
- Chekhov's Gun: The cut on Zoe's hand.
- Commonality Connection: Like Zoe, Ali was a lonely child prodigy working in an isolated base. Subverted. Her backstory was faked to gain Zoe's trust.
- Foreshadowing: Early on, Zoe thinks Ali's description of their first meeting sounds odd. Before The Reveal, an alert listener may well have noticed that Ali isn't actually mentioned in Zoe's narration, and that she keeps interrupting to say "And I was there too".
- Friendless Background: Zoe lives alone, and has no friends or family, which she puts down to the damage to her memory.
- Gone Horribly Right: The Acromatics do purge the subject of the infections, but they also absorb the healthy DNA in the process.
- Groin Attack: Performed accidentally by Zoe on Jamie, when they're forced to hide in a wardrobe.
- Hoist by His Own Petard: The director of the institute is killed by the Acromatics, and the Acromatics themselves are killed when the Doctor tricks them into absorbing a fatal virus that infects the rest of them.
- Impersonating an Officer: In the time it takes for Zoe to be captured and interrogated, the Doctor has convinced everybody that he's an agent of the Company and Jamie and Zoe are his subordinates.
- Meaningful Name: Also a Shout-Out; the Achromatics are named for Dorian Gray.
- No Plans, No Prototype, No Backup: Enforced by the Doctor, Jamie and Zoe; they destroy all records of the Achromatics before their departure. However, the Company have realised there's still a copy of the plans in Zoe's eidetic memory.
- Screw the Money, I Have Rules!: Zoe refuses to give the company information about the Achromatics' genetic structure even if they offer to help her with her memory issues.
- Sequel Hook: At the end of the framing story, an offer is made to Zoe. Her memory restored in exchange for her knowledge of the Achromatics. She rejects it, and wonders if the Company might come back with a better offer... or turn nasty.
- Spotting the Thread: Zoe's most distinct clue that Ali is lying is when she recalls a man saying that five more corpses won't make a difference when Ali was allegedly in the room as well to make it six.
- Unreliable Narrator: Not Zoe, but Ali; she was never there, and the passages she narrates never took place.
