Peter Capaldi makes his final appearance as the 12th Doctor, who faces a run-in with his own past. The Doctor's recent battle with the Cybermen has left him stranded in an Arctic wilderness, at the brink of regenerating, and confronted with his first incarnation, who is also facing his last stand. The two Doctors must battle a mysterious glass creature with the power to freeze time, and unravel the mystery of a First World War army captain transported far from the battlefield. Pearl Mackie returns as companion Bill alongside guest stars Mark Gatiss and David Bradley, while the climax features the first glimpse of Jodie Whittaker as the new Doctor.