Doctor: Jodie Whittaker
David Bradley – First Doctor
Colin Baker – Sixth Doctor
Peter Davison – Fifth Doctor
Paul McGann – Eighth Doctor
Sylvester McCoy – Seventh Doctor
Jo Martin – Fugitive Doctor
David Tennant – Fourteenth Doctor
Sacha Dhawan – The Master
Jemma Redgrave – Kate Stewart
Jacob Anderson – Vinder
Bradley Walsh – Graham O'Brien
Patrick O'Kane – Ashad
Joe Sims – Deputy Marshal Arnhost
Sanchia McCormack – Train Marshal Halaz
Danielle Bjelic – Curator
Anna Andresen – Alexandra
Richard Dempsey – Nicholas
Jos Slovick – Messenger
Nicholas Briggs – Voice of the Daleks and the Cybermen
Barnaby Edwards, Nicholas Pegg – Dalek Operators
Simon Carew, Jon Davey, Chester Durrant, Mickey Lewis, Felix Young, Richard Price, Andrew Cross, Matt Doman – Cybermen
Bonnie Langford – Melanie Bush
Katy Manning – Jo Jones
William Russell – Ian Chesterton
The Doctor and her companions rescue a bullet train in space from a CyberMaster attack, but Dan is nearly killed during the escapade. Having developed a sense of his mortality, Dan parts with the Doctor to get back to living his life. A renegade Dalek contacts the Doctor, offering information about a Dalek plot to destroy humanity.
The Doctor's former companions Tegan Jovanka and Ace investigate the abduction of several seismologists and addition of Rasputin's face to fifteen paintings. Kate Stewart invites the Doctor to UNIT's new headquarters to meet Tegan and Ace and pass on their findings. The Doctor clarifies that the new face in the paintings is the Master's, whom she goes to confront.
The Doctor learns the Master and his CyberMasters have allied with the Daleks and enslaved an energy being called the Qurunx, to end humanity by triggering eruptions from all volcanoes on Earth simultaneously. The Doctor meets the renegade Dalek, discovering the other Daleks had allowed it to contact her in order to capture her, which they do after killing the traitor.
UNIT arrests the Master, who reveals he pretended to be the Doctor and sent to Tegan a miniaturised Ashad, who enlarges and acts as a portal to bring troops of Cybermen into UNIT HQ, freeing the Master. The Daleks take the Doctor to 1916 Russia and hand her to the Master, who uses Gallifreyan technology to force her to regenerate into him. In her mind, the Doctor encounters manifestations of her First, Fifth, Sixth, Seventh, and Eighth incarnations, who inform her it may be possible to undo the Master's forced regeneration with help from the outside.
An AI program the Doctor created, using images of the Fifth, Seventh, Fugitive, and Thirteenth Doctors, leads Ace to meet with fellow former companion Graham O'Brien and destroy the Dalek volcano machine, Tegan to destroy the Cyberman converter and CyberMasters in UNIT HQ before they convert Kate, and Yaz and previous ally Vinder to capture the Master and force him to undo the Doctor's transformation. Defeated, the Master mortally wounds the Doctor with the Qurunx's energy beam, causing her to begin regenerating.
The Doctor takes Yaz home before departing alone. Yaz attends a support group of former companions including Graham, Dan, Kate, Ace, Tegan, Ian Chesterton, Jo Jones, and Melanie Bush. The Doctor then travels to a cliffside beach, where she regenerates into the Fourteenth Doctor, who is physically similar to his tenth incarnation.