Doctor Who: How did Missy become Sacha Dhawan's Master?

It made for a jaw-dropping closing sequence to the episode, but once we got our breath back, two rather large questions posed themselves... 1) how exactly is the Master still alive? and 2) weren't they sort-of a good guy last time we saw them?

In case you need a refresher, the last time we saw the Master – then going by the moniker 'Missy' – she'd been shot down by a previous incarnation, John Simm's Master, and left for dead.

Doctor Who's 10th series had seen Missy (Michelle Gomez) undergo something of a redemption arc: after the Doctor fought to have her spared from execution, Missy attempted to become "good" and though she was briefly tempted by the wicked ways of her past self, she eventually turned on Simm's Master to ally herself with Peter Capaldi's Doctor.

Furious, the Simm-Master - who'd been stabbed in the back by his successor, figuratively and literally - gave her "the full blast" from his laser screwdriver. He implied that this would inhibit her ability to regenerate and Missy apparently died shortly afterwards, having finally found absolution.

How did she survive? The hard truth is we may never know... and in the grand scheme of things, it's not really all that important.

Ever since he first appeared in Doctor Who in 1971, The Master has had a history of avoiding apparent certain death with little-to-no explanation: Simm's Master refused to regenerate and 'died' in 2007 episode Last of the Time Lords, only to be revived and return several times afterwards, while earlier incarnations (as played by Roger Delgado and Anthony Ainley) would similarly defy the Grim Reaper time and again.

• Listen to RadioTimes.com's Doctor Who podcast: Spyfall – Episode One review

It is interesting to note, however, this new Master appears to indicate that he 'stole' the form of the original O, whose miniaturised corpse he now keeps stowed away in a matchbox – so perhaps Missy didn't experience a straightforward regeneration into Dhawan's version, but something more akin to the body-snatching we saw the Master perform back in the classic series?

Left a 'walking corpse', an earlier version of the Master (Geoffrey Beevers) was able to possess the body of Tremas (Anthony Ainley) in the 1980s, while a paramedic named Bruce (Eric Roberts) also fell victim to the villain's powers of possession in the 1996 Doctor Who TV movie.

The bigger question, though, and one we hope does get answered, is why the Master has returned to his old villainous ways after Missy apparently found redemption. Some fans have even complained that the Spyfall - Part One twist has "undone Missy's poignant death" and "regresses the character", while offering "absolutely no explanation as to how she survived".

In fairness, there was hardly time or space in the episode's breakneck final scene for a long-winded explanation as to why Missy/the Master has reverted to form – possibly there's a lot more to come on this front in Part Two.

It's possible, too, that we may actually already have a clue as to what triggered this 'regression' – could it somehow be related to the Master's insistence that “everything [the Doctor] think[s] [she] know[s] is a lie”?

• Doctor Who hints at big new mystery – what is the Doctor’s lie?

If Missy/the Master had survived long enough to discover some great secret that up-ended everything they thought they knew about the Doctor, could it have been enough to destroy the fragile trust built up between the two? Having finally decided to plant their flag firmly in the Doctor's camp, having that trust betrayed might have been enough to send the reformed villain hurtling backwards...

It’s a theory at any rate. But whatever the cause, it was inevitable that this was the way things were going to go – though Doctor Who's aesthetics are changeable, many of its core concepts are immutable. The Doctor is a hero who travels through time and space in a blue box... and the Master is her "best enemy" – one way or another, a return for the character was always going to see them back up to their old, evil tricks.

Doctor Who continues this Sunday at 7pm on BBC One