In Ireland during the early 20th Century, a young couple adopt an abandoned baby and name him Brendan (Evan McCabe). He grows up to be a policeman and falls off a cliff fighting a criminal but somehow survives without injury. He retires years later, but his adoptive parents, who have not aged, wipe his memory.
In the far future, the last survivors of the human race are hunted nearly to extinction by the Cybermen. A pair of cyber-shuttles are in the air and the Doctor (Jodie Whittaker) arrives with her companions Graham (Bradley Walsh), Ryan (Tosin Cole), and Yaz (Mandip Gil). But they are unable to protect the survivors and some of the humans die. The Doctor orders her companions to depart in the TARDIS with the last survivors, but Ryan and Ethan (Matt Carver) are separated by the Cyberleader Ashad (Patrick O’Kane). Ryan and Ethan escape and help the Doctor hijack a cyber-shuttle, which is pursued.
In deep space, Yaz, Graham, and the other humans, Yedlarmi (Alex Austin), Ravio (Julie Graham), and Bescor (Rhiannon Clements), discover a battlefield covered in dead Cybermen. They board a huge cyber-carrier, hoping to be transported to Ko Sharmus, a haven for humans at the Boundary, a portal to the other side of the universe. Inside, they find thousands of new Cyberman warriors in stasis, just as Ashad and his crew dock their shuttle.
The Doctor, Ryan, and Ethan arrive at Ko Sharmus, which is actually a person acting as ferryman to the Boundary, and a portal opens. Ashad and the Cybermen are resurrecting the warriors. Graham and Ryan are on the control deck. Ashad comes with warriors as Yaz contacts the Doctor and warns her. The Doctor watches in horror as the portal reveals the ruined Gallifrey, where the Doctor discovers the Master (Sacha Dhawan).
The episode was written by Chris Chibnall and directed by Jamie Magnus Stone. It was well received with a 93% approval of the critics, to be continued by the next episode, The Timeless Children.