Doctor Who: 10 Huge Questions After Spyfall Part One
6. Who Can We Trust?

The Master’s ‘last words’ to the Doctor was that everything
she thought she knew was a lie. Given that she’d fallen for his Horizon Watcher
identity, the implication is that even with her nemesis unmasked, all is not as
it seems. There are other lies, hidden in plain sight. Could this play into a
series long arc, or even hark back to the unanswered Timeless Child references
of the previous series?
The Doctor tells her companions to trust nobody. It is a lesson she might have to learn for herself. For starters, is the Yaz who materialised in Australia the real Yaz? Can the TARDIS even be trusted? It is worth recalling here, C’s comment about Daniel Barton possibly being a double or even a triple agent. Could the same apply to O and the Master? Is it possible that he is neither of those people? What if this is another form of the aliens, this time sent to mock the Doctor rather than the humans?
There are certainly more twists in this tale to come, but all the signs are present for this becoming a fairly typical Master tale. The character makes an alliance that he soon regrets and the two Time Lords end up needing to help each other. The Master, for all his talk of wanting to kill her, may be warning the Doctor, giving her a clue about how to survive the new reality she is transported into.