A solar tsunami sends the TARDIS hurtling towards a futuristic factory on Earth, where human doppelgangers are used to mine dangerous acid, as the time-travelling adventures continue.

A second wave hits and the "Gangers" separate. They can remember every second of their "original's" life and feel every emotion they've ever experienced. But are these memories stolen or have they been bequeathed? Are the Gangers merely faulty machinery that must be shut down or are they living, breathing, sentient beings? Can the Doctor convince the terrified humans to accept these "almost people" and prevent an all-out civil war before the factory explodes?