This is my theory. For a number of reasons. All little things really, but when you piece them together, you're left wondering. Or rather, I am! Sorry if someone's already posted this as a theory.

1) Most obvious- she acts a bit like him. I think so anyway. Or at least, how he would act if he was a female. Still mad and manic, but obviously much more feminine like. What's more, the clothes Missy is in remind me of Anthony Ainley's cotsume. As to why Missy would describe the Doctor as her boyfriend, well has anyone ever sarcastically described someone they despise as a friend?

2) Rumours- there were rumours about the Master's return back when the series began, but they just kind of fizzled out. Of course, a rumour is a rumour, not to be taken as evidence, but at the same time, no smoke without fire...

3) Timing. The whole debate about Time Lords changing genders has been especially strong during the new series, and Moffat's era in particular. What with it being confirmed as possible on screen a couple of times recently(ish), it's now entirely within the rules that a female Doctor could one day be cast. Of course, Moffat didn't want or need to do this, what with Peter Capaldi being readily available, and excellent for the role. But Moffat likes to experiment, and try new things with Doctor Who (see series 6 for further details). Would he cast a female Master? Yes, I think he would. Just to see what happens, and to try and prove that this could, in theory, one day work for the Doctor himself.

4) Is he/she working with the Cybermen? He once worked with the Daleks (sort of), so why not?

5) There seemed to be a lot of emphasis on the Doctor being the last of his kind again in this episode. Even though he probably actually isn't, with Gallifrey safe but lost.

6) Clara could indeed be hypnotized next week, judging by her behaviour from the trailer. The Master used to specialize in this.

Those are most of the reasons. I don't actually necessarily think that Missy IS the Master. I'd be quite shocked if that actually turned out to be the case. But I do think it's a plausible possibility. I'm not sure about her being Clara. I don't see why Steven Moffat would want to make this finale connected to his previous one. He's a creative man. I really hope that isn't what's happening, because my family, as casual viewers, wouldn't have the first clue what happened in that episode, and I'm sure many other casual viewers wouldn't either. It was a great standalone episode, but it wasn't one that is in any need of a sequel.

So yeah. I'm very much expecting this to be false, but I do think there's basis for considering it. I do hope it is true. Much more exciting that Missy being a version of Clara.

Missy being The Master would seem quite likely (if only because the female version of a Master is a Mistress).

....but I'm leaning towards Missy being The Doctor/an alternate version of him...or The Rani..or Clara..or River...or K9.

Missy will be a new character I am certain.

She is the rani

I definitely think she's someone we know, even if it's just some evil iteration of Clara. The "you know who I am" line in the trailer sealed it for me.

That has no bearing on whether we know who she is or not, though. It could also be abstract, as in: "You know who I am... I am Chaos, I am the bringer of Death, I am your worst nightmare..." kind of thing.

We simply don't know enough to know that yet. And I, personally, am leaning toward her being completely new to us, the audience.

I'm not even sure she is a bad guy yet we haven't really seen her do anything bad she has talked to a few dead people both bad and good hinted she picked Clara and watches The Doctor and Clara but nothing to say she is evil it may turn out she is a good guy.

Maybe Clara is the rani. Didn't see that one coming.

If she is, we'll have to congratulate the person who first suggested it, you know all those months ago, in all those other threads ....

I have a horrible, sinking feeling she is the Rani. I really didn't want to see her back again.

There was also the mention of 'old enemies.' I know the Cybermen are old enemies, but Missy might be too.

Wouldnlt it be good if one of the 'old friends; who helps is the Curator?

no she isnt.

To be honest, I don't spend very long on these forums, ad I certainly can't remember what people were posting a few months ago. Do you intend to across as quite sarcastic and rude, because I'm afraid to say you do a bit. Which puzzles me, because there was no reason for it.

But yes, congrats must be given to anyone who guessed it months ago.

Yesssss & congratulations to you on your post, as i too get really annoyed with some other posters who bemoan members for not remembering posts or threads from days, weeks, months or even years ago. I'm glad someone else feels like that too.

Agreed.

its too obvious and at the same time ridiculous.

I don't think missy is the master though. For me it would be too obvious. However, I do think we will see the master return to come face to face with Capaldi not now but in the next series. I had thought Danny Pink might be the master but I believe differently now. Danny has an edge we have had glimpses of but suspect now this is due to an undercover character re: Clara.

Indeed I'm very sarcastic and rude in almost every post I make, I make it my habit indeed, in fact the only reason I come on to the forum is to be rude to everyone, does that make it easier to think that way of me? Everyone is just a critic eh, not allowed to voice your opinion because someone will make a snide remark?

There is a search button on the forum, thats all I was saying.

I was agreeing with the sentiment of the original post. Not a dig at you Judge. Fairly new to the forum and have encountered a few offensive posts re these issues. Have also read some of your posts and not once felt you were offensive. So my bad if you thought it was directed at you.

No probs, none taken. Its silly to be offended at postings on a forum eh

p.s. I've enjoyed your posts new to the forum or not

Well, congratulations to people who guessed right! I was one of those people, but I'm not so arrogant as to congratulate myself lol.

No, Missy isn't the Master. She's a robot who thinks she is the Master.

If she is the Master, they're going to have to explain very quickly how he/she got back from Gallifrey after disappearing with it at the end of David Tennant's reign.

Did The Master end up back on Gallifrey? Was never actually confirmed.

Well I won my bet with the kids. I told them Missy was going to be the master. Thats a jelly nany they owe me....
There has been so much talk over the years of having a female doctor, but I said it was more likely to be a female master next. As others have said i wonder if they will explain what has happened since we saw him/her at the end of Tenants era.