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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 the Spy Master'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) The Doctor, however, suspected that the Master ultimately escaped the Kasaavin, but was unable to find him. (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)
