Parts: 1 (60-minute)
Writer: Steven Moffat
Airdate: 25 December 2017
Production Code: n/a

Doctors: twelfth, first
Companion: Bill

Story: Ypres, 1914: A tense standoff in an impact crater comes to an abrupt halt — literally — when time freezes around a war-weary British captain. As he explores the unnaturally-still battlefield, the captain encounters a mysterious figure made of glass... who abruptly transports him, floating in and out of consciousness, to a futuristic chamber where an unfathomable procedure commences. But before the captain can get a sense of where he is, the loud announcement of a "timeline error" interrupts the procedure and he finds himself in —

Antarctica, 1986: where the first Doctor, determined to hold back his incipient regeneration, stumbles upon the regenerating, and equally determined, twelfth Doctor. Though both Doctors are trying to stave off the inevitable, the twelfth Doctor soon realizes that if his original incarnation succeeds in dying rather than regenerating, it will create the most catastrophic paradox.

And then time freezes around them, and a World War I British captain, dazed and fearing himself pursued by the mysterious glass woman, staggers out of the suspended snowfall to solicit their assistance...