The evidence is stacking up as to who or what is locked behind the mysterious vault that the Doctor is sworn to guard in Season 10 of Doctor Who.

Potential spoilers incoming for your past, present, and future of Doctor Who ahead.

Considering the tease that John Simm’s Master was returning to the show this season, that version of the rival Time Lord seemed the likely culprit for imprisonment. That was especially the case when, towards the beginning of “Knock Knock,” we heard someone knocking on the door, which is a motif often associated with the Master. But recent evidence suggests that locked inside is actually the female incarnation of the Master, Missy (Michelle Gomez), instead.

Though it’s believed that viewers will get some vault answers in the upcoming fifth episode — “Oxygen” — there is no actor unaccounted for on the credits. But Michelle Gomez is slated for a return in the following episode, airing on May 20: “Extremis.” Furthermore, in an early trailer for Season 10, Missy was glimpsed laying atop something in the same pose one would lay across a piano … which is the exact instrument we heard at the very end of “Knock Knock” when the Doctor decided to enter the vault.

Missy is slated to return this season. Is she in the vault?
Missy is slated to return this season. Is she in the vault?

Whether or not it happens in the next episode, we’re bound to find out very soon just who is in the vault, especially if the Doctor has started willfully entering.

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Doctor Who airs Saturdays on BBC and BBC America.

Photos via 'Doctor Who' / BBC (1, 2)