- She has an accent similar to The Doctor and, upon hearing his Scottish accent, remarks that she thinks she'll keep it
- She calls The Doctor "her boyfriend"
- She calls Clara "My Clara"
- She hints at being responsible for keeping them together (Lady in the shop?)
- She operates in a place called The Promised Land, where people appear to show up after they die
Given those tiny scraps of information, there are some popular fan theories floating around. Here we examine five of the most popular theories and rank them from least to most likely. There are some popular theories we've decided not to pursue here, because they either seem too unlikely (e.g. Romana) or because we'd just be too annoyed if they came to pass (e.g. River Song).
5. Future Clara
What: This theory explores the idea that Clara somehow becomes Missy (Or is that Miss C?). We're not sure if this is simply referring to an aged version of Clara or some sort of change she undergoes as a result of one of her adventures with the Doctor and timey-whimey Tardis properties.
Why: There are a few concepts to support this theory. In the episode "Flatline," The Doctor seems disturbed by Clara's bold behavior and the way she ignores the loss of human life for the first time. He tells her she was an excellent Doctor, but that being "good" had nothing to do with it. We also know the Tardis was weirdly resistant to Clara the first few times she tried to board it. Lastly, Missy refers to The Doctor as "my boyfriend," which could be a snarky reference to their awkward relationship shift and the way The Doctor repeatedly told Clara he was "not her boyfriend" at the beginning of the series.
Why Not: Well, this one is fairly obvious. As much as we love the idea of seeing a companion evolve into a villain due to her time with The Doctor, this probably isn't going to happen. They don't look enough alike. They both are women with brown hair, but the resemblance ends there. Clara's eyes are brown, Missy's are blue. Unless some kind of odd regeneration-timey-whimey thing happens, it wouldn't make enough sense.
4. The Rani
What: The Rani is a Time Lord from the early series who was exiled from Gallifrey for carrying out immoral scientific experiments. Inhabiting the role of evil scientist genius, The Rani prioritizes her research above else. She and The Doctor had only a few run-ins before the original show was cancelled.
Why: The Rani is a Time Lord with a Tardis. That makes it easier to explain the "Promised Land" in which we see her operate - she could be operating from her own Tardis and somehow transporting those who are about to die during the last second of their lives (or after). She has a history with The Doctor, so her observation of him and her seeming vendetta against him already makes sense. She wasn't necessarily on Gallifrey during The Time War, so her whereabouts are unknown and could be explained. Lastly, her new appearance would also make sense because she has the ability to regenerate.
Why Not: The Rani is a fairly obscure villain, so if Moffat wanted to pull from and add his mark to classic Who, this may not be the best choice. Also, we're not sure whether this leans for or against this theory, but Big Finish, who produces audio books of classic Doctor Who stories, recently announced her character would be appearing in a new audio production in December of 2014. Though the coordination would be interesting, it isn't standard protocol for the audio books and on-air new-Who to align like that.
3. The Doctor
What: This theory points to Missy being some sort of version of The Doctor. We're not sure whether that means a future regeneration or some kind of element of The Doctor's personality (perhaps a la The Dream Lord). This could also be a consequence or side effect to the new set of regenerations The Doctor received.
Why: The most pointed evidence to this is what we've seen of Missy's behavior and what she says. Most of her comments - about choosing Clara, about her being "My Clara," and about keeping the new accent, seem to suggest she either has some level of control over The Doctor or somehow is The Doctor. This would also be in keeping with the theme of this season, which has explored who The Doctor really is (and no, we don't mean his name - we mean his personality, the part of it that endures beyond each regeneration).
Why Not: We've seen stills from the set of the two-part finale, and we know The Doctor and Missy interact with one another. Though past and future regenerations have interacted, it doesn't seem very likely the show would tread this territory again so soon.
2. The Valeyard
What: The Valeyard is another classic Who villain and is also technically also an extension of The Doctor, albeit a more specific one, so we've put it in its own category. The Valeyard is an amalgamation of The Doctor's darker and potentially more evil characteristics. The Valeyard was said to show up between The Doctor's 12th and final regeneration.
Why: We know Moffat hasn't ignored or rejected The Valeyard; it was mentioned in The Name of The Doctor as one of the names The Doctor would be known by. The timing is also right, given that The Doctor just ended his "final" regeneration and is now on a new set (depending on how you count the War Doctor, we could buy that the Smith to Capaldi regeneration would be an acceptable time for The Valeyard's appearance). Series 8 has spent a lot of time pondering whether The Doctor is good or evil, so a physical manifestation of The Doctor's evil qualities would be in-keeping with that theme.
Why Not: If anything, Capaldi's Doctor has been darker than the ones we've seen before. At first this resulted in speculation that Capaldi would actually be the Valeyard, but so far his actions have on the whole been more positive than negative (in terms of saving lives and helping others). If The Valeyard was intended to show off The Doctor's dark side, but isn't Capaldi, it's hard to see why they'd spend so much time developing The Doctor's dark side this season.
1. The Master
What: The Master is a Time Lord who has been played by 8 different actors over the course of the show. He's appeared in both new and classic Who and is at-odds with The Doctor, who was once his childhood friend. We last saw The Master played by John Simms during Russell T Davies' run, in which it was revealed that The Master was being used by the Time Lords in an attempt to escape destruction. In this theory, Missy is his latest regeneration.
Why: Master, Mistress - Moffat has a thing for the play on words and hiding in plain site, so that seems like an indication. We've been saying all along that this iteration of The Doctor is Batman, and every classic hero needs a classic villain. If The Doctor has a "Joker" style villain, recurring, unpredictable and a bit crazy, it's got to be The Master. When we saw him last, he was driven mad by a constant drumming in his head. It would also be an interesting way for The BBC to gauge reaction to a potential female Doctor in the future, as The Master has always been a male character.
Why Not: Missy's own comments are the chink in this argument. References to keeping the accents, "my boyfriend," etc. don't really line up with this theory. It also appeared as though The Master was pulled back into the Time War in the episode The End of Time. Because all Time Lords are still trapped in stasis, it will take some acrobatic writing to pull The Master into the same time and place as The Doctor.
It's also a bit predictable, and Moffat has a tendency to lean towards new ideas that are harder to guess. Which brings us to Theory 0...
0. Something New
Because in Moffat's scripts there's always some surprise extra option, we bring you Theory 0. This is the most obvious and probably most likely scenario. Missy is something new, unreliant on canon, that will be completely impossible to guess prior to the finale. Moffat has a penchant for creating his own, new villains, and since The Cybermen play heavily into the finale, it wouldn't be surprising to see Moffat create a new nemesis to help them. But hey - speculation is still fun!