This article contains spoilers for Doctor Who: Flux episode 6.
Viewers can be forgiven for suspecting the Grand Serpent, one of the villains of Doctor Who: Flux, is a Time Lord — but that's not the case. When he made his debut in Doctor Who: Flux episode 3, most viewers assumed Craig Parkinson was simply making a cameo as a corrupt official. He appeared in Vinder's flashbacks when he was plunged into his own personal timeline in the Temple of Atropos, with Vinder banished when he attempted to expose the Grand Serpent's corruption.
The Grand Serpent unexpectedly returned in Doctor Who: Flux episode 5, an ally of the Sontarans who helped them invade Earth by neutralizing UNIT, Earth's primary defenders against extraterrestrial invasion. There was immediate speculation he could be a Time Lord, potentially even the Master given his predilection towards dapper black suits and his ability to travel in time. Even the Grand Serpent's MO felt like a traditional Master plot, particularly reminiscent of Roger Delgado's incarnation of the Renegade Time Lord. Some viewers even suggested the Grand Serpent's favorite way of killing, an apparently ectoplasmic serpent, could be a reference to the serpentine ectoplasm form the Master briefly adopted in Paul McGann's Doctor Who movie back in the 1990s. One scene in Doctor Who: Flux episode 6 possibly supported this, with the Doctor saying she detected two pulses.
That is, however, a misread of the scene; the Doctor senses two hearts, but doesn't reach the conclusion the Grand Serpent is a Time Lord. Instead, she guesses he's a "binary demi-species, something really rare," with an ability she describes as "inter-body bio-projection." It seems the Grand Serpent's snake form is a separate body, its consciousness shared with his humanoid form. The Doctor sensed the hearts of the Grand Serpent's two bodies beating.
The Doctor's interrogation scene was one of the strongest moments in Doctor Who: Flux episode 6, with the Doctor employing a classic Tom Baker/David Tennant trick of simply babbling to distract her enemy. Parkinson and Whitaker had a great dynamic, with the Grand Serpent growing increasingly frustrated because he sensed the Doctor was gradually taking control of the conversation. The Grand Serpent ultimately attempted to use his snake-form on the Doctor, only to learn the technique didn't work because the Doctor was split across time and space — a revelation that clearly confused him, even more so when another Doctor arrived on the scene.
The Doctor's ability to identify the Grand Serpent is actually a crucial part of the plot, because it seems bio-projection duo-imaging is "subject to very painful sonic locking." This means the Grand Serpent is vulnerable to sonic waves — very useful indeed given the Doctor has a sonic screwdriver, and the Sontarans apparently use sonic restraints to hold their prisoners. The Grand Serpent's very abilities were also his greatest weakness — a constant theme in Doctor Who.