WARNING: The following contains major spoilers for the latest episode of Doctor Who, “Spyfall, Pt. 2” which premiered Sunday on BBC.

In "Spyfall, Pt. 1," the Doctor uncovered an interplanetary threat from a race of alien spies working in league with her nemesis, the Master. The villain, who first posed as a human analyst and old friend of the Doctor codenamed "O," revealed himself at the conclusion of the episode. The twist came as quite the shock, given that the last version of the Master, Missy, had somewhat redeemed herself as more of a heroic frenemy. Twisted and cruel, this new Master absolutely enjoys his villainy, and he has made it his mission to, once again, try and kill the Doctor.

This is very much a return to the Master of old. His rivalry with the Doctor even receives a boost from a throwback to a time when the two -- then played by John Simm and David Tennant -- were mortal enemies.

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Doctor Who Spyfall Part 2 The Master

In the second part of the Season 12 premiere, Jodie Whittaker's Thirteenth Doctor finds herself in 1943 Paris, desperately trying to get back to the present without her TARDIS. Hunted by the Master, she baits him into coming to her. To contact him in this time period, she uses a Morse code dial, and sends a signal only her nemesis will understand: four beats, repeated twice. The sound of drums, the heartbeat of a Time Lord.

Doctor Who fans will recognize the four-note repetition as a nod to the Tennant era. When the Tenth Doctor faced Simm's Master in Season 3, the villain was afflicted with the Drumming, a condition that caused him to hear this four-beat sound constantly. It was the source of his madness and, as it turns out, a twisted ploy by the Time Lords to be saved from extinction. This drumming was repeated often during the storyline, and returned in Tennant's final adventure as the Doctor in "The End of Time."

Now, the beat makes a brief in return in Season 12, in which it weaves connective tissue between Sacha Dhawan and John Simm's Masters, and lets us know the days of Missy are truly gone. The Master is back. The real Master.

Doctor Who stars Jodie Whittaker as the 13th iteration of the titular Time Lord. Whittaker is joined by co-stars Bradley Walsh, Tosin Cole, Mandip Gill in a series spearheaded by showrunner Chris Chibnall. The next episode is set to debut on the BBC on Jan. 12. The new season and all previous 11 seasons will stream on HBO Max later this year.

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