Kasaavin invasion

In what the Thirteenth Doctor identified as an alien invasion, the Kasaavin, who came from another realm, planned to "take" the Doctor's universe. They, along with Daniel Barton, were unknowingly part of 's plan to destroy humanity and rain destruction across the universe. However, this plan was ultimately foiled by the Doctor and her companions, who forced them back to their own realm. When the Doctor revealed that the Master intended to betray them, the Kasaavin took the Master back with them as revenge for him using them for his own motives. (TV: Spyfall)

Background
Despite living as and the Lumiat, lives where they had sought to do good in the universe, (AUDIO: The Lumiat)  returned to his dark ways, looking down on Missy for trying to better herself and spending so much time with the Doctor. (PROSE: The Doctor vs the Master) Hacking into the Matrix on Gallifrey until he "got lost", (TV: The Timeless Children) this incarnation of the Master learned the truth of the Timeless Child. In response, he ravaged Gallifrey in its bubble universe and left the Citadel in a flaming ruin, having apparently killed all of the Time Lords.

Wanting to send a message to the Doctor about what had transpired on Gallifrey, the Master found a species known as the Kasaavin from another dimension had embedded across the universe as sleeper agents. Brokering an alliance with them, the Master recruited Daniel Barton into his plan to trap the Doctor by proposing that Barton and the Kasaavin turn the human race into hard drives. As part of the plan, the Master arranged for "The Silver Lady" to be delivered to Charles Babbage, and that it was passed on to those who would influence the development of computers through history until it ended up with Barton. The Master used his Tissue Compression Eliminator to kill a newly-recruited MI6 agent on his first day, and took his place. He was put to work as an analyst known as "Horizon Watcher", and given the codename "O" as a joke on how his superior, "C", would react to his presence. During his time at MI6, the Master crossed paths with the Doctor prior to her thirteenth incarnation. Not recognising the Master, the Doctor stayed in contact with "O" via text messages. "O" was eventually sacked by C sometime prior to 2020, and the Master went into hiding in Australia, using his TARDIS as a hideout. (TV: Spyfall)

The Invasion
By 2020, the Kasaavin began to attack spies from different agencies all across the Earth once the intelligence services started to realise their presence, leading MI6 to track down the Thirteenth Doctor. While the Doctor was at MI6's HQ, the Master personally assassinated C, and the Kasaavin forced the Doctor and her friends to flee to "O"'s alleged location in the Great Victoria Desert. Upon their arrival, "O" greeted the Doctor and Graham O'Brien. He assisted the Doctor by sharing his findings and, after a staged Kasaavin attack, travelled with Team TARDIS to confront Barton.

At Barton's party, "O" blended in with the other guests until Barton fled, making the team pursue him to an airport. As Barton was about to take off, the team ran to the plane, with "O" almost not catching up, using the excuse that he was terrible at sprinting. This triggered the Doctor's suspicion, as "O"'s file said he was a champion sprinter. After being confronted over the inconsistency, the Master revealed his true identity, even showing Team TARDIS the shrunken corpse of the real "O". He then revealed he had been working with Barton and the Kasaavin, and left Team TARDIS on the plane as he detonated a bomb in the cockpit. Alerted to the Doctor's survival by Barton, the Master followed her to 1834 London, where he held a crowd at the Royal Adelaide Gallery hostage to draw out the Doctor. However, after a brief conversation with her, the Master was forced to flee when attacked by Ada Gordon.

The Master next chased the Doctor and Ada to 1943 Paris, where he set himself up as a German officer using a Teutonic psychic perception filter to hide his non-Aryan appearance from the Nazis. Once the Doctor made contact with him, the Master met her atop the Eiffel Tower, where he revealed his real plan had been to get the Doctor's attention and persuade her to return to Gallifrey. However, the Doctor had had Noor Inayat Khan leak information to the Nazis of the Master being a double agent, and, as they came to arrest him, deactivated the perception filter to ensure his capture while she stole his TARDIS. After seventy-seven years of waiting, the Master finally caught up to Team TARDIS back in 2020, just as the Kasaavin were enacting their invasion. However, the Doctor thwarted the Master's plot and exposed to the Kasaavin the Master's plan to betray them and Barton. Furious, the Kasaavin captured the Master, trapping him in their reality. (TV: Spyfall)

Aftermath
With the psychic link between him and the Doctor still active, the Master, sometime later, was pressured into a mental conversation with her, during which they looked back on the state of their relationship and, as it were, "how it had all gone wrong". The Master once again taunted the Doctor with the knowledge he held over her, saying that he would only reveal it when he was "good and ready", and ended the chat. (PROSE: The Doctor vs the Master)

The Doctor suspected that the Master ultimately escaped the Kasaavin, but was unable to find him despite searching. (TV: Fugitive of the Judoon) The Master later appeared from the Boundary after it suddenly connected to Gallifrey, confirming that he did eventually manage to escape the Kasaavin. (TV: Ascension of the Cybermen)