The Master's Dalek Plan

The Master's Dalek Plan was one of several titles that used for his plot to steal the Doctor's identity by forcing the Thirteenth Doctor to regenerate into him. This involved an alliance with the Daleks and the Cybermen, where they in return would have their greatest enemy erased and be able to conquer Earth. (TV: The Power of the Doctor)

A day to come
Operation Divide and Conquer, a collaboration between and the Daleks, (TV: Frontier in Space) was recorded in the Masterplan Journal left behind by  under "the Master's Dalek Plan". (PROSE: The Secret Diary of the Master)

After saving the universe from the Flux, the Thirteenth Doctor was confronted by Time, who warned her that her time was "heading to its end" and to beware of the forces which massed against her and. (TV: The Vanquishers)

The plan
Following his creation of the CyberMasters and confrontation of the Thirteenth Doctor on Gallifrey, (TV: The Timeless Children) the Master sought to sully the Doctor's title throughout the universe by hijacking her body and wreaking havoc in her name. To achieve this, he enlisted the help of the Daleks and CyberMasters, winning their support by promising to remove their common enemy. The Master also took Rasputin's place in history and hypnotised Tsar Nicholas II into allowing him to use the Winter Palace for his operations, such as the setup of connected regeneration chambers from Gallifrey.

Forced regeneration
Using the power of a Qurunx, the Master ultimately succeeded in forcing the Thirteenth Doctor into the chambers and triggering her forced regeneration while his own body fell motionless in the other chamber. (TV: The Power of the Doctor)

Following the forced regeneration, the Doctor's new body had the Spy Master's appearance and personality, and he forced Yaz to be his companion by threatening her with the Tissue Compression Eliminator. Using the Doctor's TARDIS, he began to cause conflicts and destruction in the Doctor's name.

"I'm going to make the Doctor a byword for fear, pain and destruction, so when people hear that name in future, they quake in fear."

- TV: The Power of the Doctor

From a moon, he watched two planets destroy each other as a result of his plots, and he called out to a visiting hover camera that "I am the Doctor. And I caused this." However, Yaz pushed him out of the TARDIS and fled. He remained on the planet, playing a slow tune on the Doctor's recorder.

Yaz eventually returned, feigning regret for abandoning him. Together they travelled back to the Winter Palace, where he gave the orders for the Daleks and Cybermen to begin their invasion of Earth. (TV: The Power of the Doctor)

Reversal
While the Master's plan was being carried out in the Doctor's new body, the Thirteenth Doctor remained in their mind, refusing to complete the regeneration. Her refusal to change manifested itself as the Guardians of the Edge at the Edge of Existence, who explained to her that they had stopped her from passing the Edge, which would "let the Master permanently hijack [their] existence". With their encouragement, the Doctor realised that the newly regenerated body was still unstable, especially as it was forced, and could potentially be undone.

The Doctor's AI hologram picked up on this and devised a plan. The Holo-Doctor assisted the Doctor's former companions, including Tegan Jovanka and Ace, to thwart the CyberMasters' and Daleks' invasion plans. Finally, Yasmin Khan and Inston-Vee Vinder cornered the new "Doctor" and forced him back into the chamber, using the CyberMasters' regeneration energy to cause a "forced degeneration" that reversed the process, bringing back the Thirteenth Doctor and forcing the Master "back into" his body. As a result, the Master's body began to fail as he escaped back to his TARDIS. (TV: The Power of the Doctor)

Aftermath
Despite the reversal of the regeneration, however, the Master declared that if "[he] couldn't be the Doctor, then neither could [she]" and redirected the Qurunx's destructive beam in her direction, fatally wounding her and triggering a regeneration. (TV: The Power of the Doctor)

After returning Yaz home, the Doctor travelled alone to a cliff overlooking the sea, where she could watch the sunrise one last time. After saying her farewells, she regenerated into the Fourteenth Doctor who, much to his shock, realised his resemblance to the Tenth Doctor. (TV: The Power of the Doctor) Incidentally, his first trip led to him being embroiled in what appeared to be a 1966 Dalek invasion of Earth. (COMIC: Liberation of the Daleks)

Behind the scenes

 * The Master's Dalek Plan is a pun based upon the First Doctor story titled The Daleks' Master Plan. This pun had previously been used for the Big Finish War Master audio story of the same name, which sees the War Master manipulating Dalek history.
 * Through this forced regeneration, Sacha Dhawan is the first actor to have portrayed both the Master and the Doctor within the same story.
 * As this forced incarnation came directly after the Thirteenth Doctor, Sacha Dhawan technically portrayed a possible version of the Fourteenth Doctor, although the Thirteenth Doctor's conversation with the Guardians of the Edge also makes clear that the Doctor's actual next regeneration (as played by David Tennant) was supposed to be a separate individual.