This is a one shot, but if you like it I might continue it as a chapter series! Additionally, I've said that both the Master and Missy are in this as finally Missy has a character! But the Master is not in it - I will remove his character tag once people are more aware of Missy's new character. Set after Lie of the Land. *UPDATE: this chapter has been rewritten* - W

"I thought you had some kind of monster in here, or something!"

Bill frowned as she sat in the TARDIS with the Doctor. The Doctor was examining something on the monitor. "What?" He asked without turning to face her.
"What, what?" Bill replied nonchalantly.
"Your frown is practically audible." The Doctor replied. Bill chuckled.
"Its not very important." She said, shaking her head. The Doctor turned to face her and leaned against the console, his eyebrow raised. "Okay, okay!" Bill sighed, holding up her hands in surrender. "I just... I was thinking about the woman in the vault. Missy." She said quietly, sensing the Doctor's change in mood as the subject was brought up. The Doctor hummed and nodded for her to continue. "I wanted to know more about it all. Why you have a woman in a vault, first of all. Who she is." Bill explained, biting her nails nervously. The Doctor stayed silent for a moment.
"I'll tell you later. I have to do some TARDIS repairs, first." He replied, smiling slightly. Bill sighed, knowing that the Doctor was changing the subject.
"Alright." She said.

Later, while the Doctor distracted himself with TARDIS repairs that he likely did not need to be doing, Bill found herself at the vault. She wasn't sure why. Maybe she thought that Missy was more likely to give her the answers she wanted, or perhaps it was pure curiosity. Whatever it was, it was on arrival that she realised her visit had been in vain. Staring at the huge, multi locked doors, she wanted to face palm. "It's locked. Of course it's locked." She whispered frustratedly. She pressed her ear against the door and wondered whether Missy could hear her. She slid to the floor and sat with her back to the doors.
"I wish I understood more than I do." She muttered. She pictured the Doctor finding her sitting at the door of the vault and shook her head, deciding that was not something she wanted. "Why am I here..." she sighed, rolling her eyes.

It was a couple days later when Bill found herself at the vault again. The same feeling of self doubt filled her as she wondered why she had come back to the vault when it contained a so-called "monster". The huge, locked and unwelcoming doors were her only company once more. "I don't know why I'm back here." She said aloud. She sighed as loneliness crept into her heart, and pictured her mum beside her. "Hello mum." She smiled. Her mum smiled in response, but as usual kept silent. Bill didn't mind. Her image was enough. "I know I already told you about how you saved the world. How we saved the world, I guess. I'm not going to keep talking about that today." Bill explained. Her mum nodded, gesturing for her daughter to continue.

"I miss you, mum. It's dumb, I guess, because I barely even knew you. But I miss you." Bill said quietly. Her mum smiled sadly, as if she wished to reach out and stroke her daughter but could not. "I've been thinking..." Bill began, before stopping as a noise entered her ears. Very, very faintly, was the sound of a piano being played. A sound that was coming from behind the huge vault doors. Bill frowned as she listened to the source, and didn't notice as her mum's image faded away.

"Missy?" She said quietly through the door. The piano didn't stop, and there was no response. Bill thought the song sounded like 'The Entertainer' by Scott Joplin, however she wasn't sure - she never had been a music expert. She found herself humming along quietly after sitting there for another few minutes. "She's pretty good." She mumbled to herself.

The third time she visited Missy's vault, Bill felt ashamed. She felt as if she was hiding a secret from the Doctor - which technically she was - and didn't even have an explanation as for why. There was no piano when Bill arrived, only silence as her company. She sat against the door once more, and pulled out a book the Doctor had asked her to study - something about black holes. She read a couple mind-numbing sentences before throwing the book down beside her. "This is ridiculous," Bill muttered, "I wish he'd just answer my bloody questions instead of giving me these useless books."
A whisper suddenly came from behind the thick door, and Bill momentarily questioned how it carried through the material. "I would answer you." it spoke enticingly.
"Missy?" Bill whispered in reply, shocked. The voice did not come again.

When the Doctor continued to ignore Bill's questions, Bill began to lose her patience. "Don't you trust me?" She snapped, after he denied her for what felt like the hundredth time.
The Doctor turned to face her with a frown. "Of course I trust..." he began. Bill held up her hand to silence him.
"Then how come you won't answer me?" She interrupted. The Doctor hesitated and shook his head.
"Because some things don't need to be relived. I'm sick of discussing it." He said, clearly attempting to dismiss his companion. Bill groaned in frustration and stormed out.

She was unsurprised to find herself at the vault once more. It had begun to be a place in which she could escape - no one would find her down here, not even the Doctor, as he would never expect her to voluntarily sit outside the vault. Bill sat against the doors and sighed, feeling sorry for herself. She wondered whether the Doctor wouldn't tell her because she was only a human. "A stupid human." Bill sulked quietly.

Suddenly, the huge doors of the vault groaned and slowly opened inwards, causing Bill to fall backwards slightly, catching herself before she embarrassed herself too much. The time lady she had met not long ago stood in front of her, well dressed and put together as ever. She looked down at Bill with a frustrated expression. "Stop your moping, dear, and come inside." She said impatiently.
Bill frowned. "I'm not moping." She snapped, incredibly confused with the entire situation. Missy laughed as if Bill had said something hilarious.
"Come in." She beckoned, opening her arms wide in an over exaggerated welcome to her vault.
"How did you... how did you get out?" Bill asked, gesturing to the doors. Missy tapped the side of her nose.
"Remember what I said about the leaf gun earlier? How I made a gun from leaves? I'm very clever dear." Missy explained, slowing down her words as though Bill was stupid.
"Does the Doctor know?" Bill asked, following her inside. Missy cackled.
"Does he know I'm very clever? Yes." she answered.
"Then why would you stay?" Bill queried, confused.
Missy frowned as if she had eaten something that tasted disgusting. "I'm... being good." She said unhappily.

Bill nodded slowly, and followed Missy to the chairs by the fireplace. She hovered near the chair, unsure of whether to sit down, until Missy rolled her eyes and sighed "Come on, pet, I don't have all day". Missy lounged comfortably in the chair, completely making herself at home in her prison. "You want to know things." Missy stated. Bill suddenly remembered that Missy was meant to be inside the containment field, but decided she couldn't be bothered asking about it.
"Basically." Bill replied uneasily. After Missy's silence, she added "I don't think he trusts me. The Doctor, I mean."
"Well, he definitely trusts you, dear. He always trusts you humans far too easily. It's disgusting." Missy replied, her words more comforting than expected. Her eyes softened slightly. "He doesn't not answer because he doesn't trust you." Missy said. Bill remained silent, mostly out of confusion. "He's old and sentimental. We've lived a long time, me and him. Him and I. Sometimes bad things happen. And when they do, the Doctor chooses to lock those memories away. As if that'll hurt less." Missy explained. She rolled her eyes on the last sentence, as if it was ridiculous.
"And what do you do?" Bill asked. Missy frowned in thought.
"I live with them." She said after a while, surprising even herself with her honesty. Bill nodded.

"I wanted to know why you're here. And who you are." Bill asked hesitantly.
"I'm Missy. Other last of the time lords." Missy replied with a disturbing smile.
"No, I know that. The Doctor said that. But who are you to him? And what... why did he call you a monster?" Bill asked.
"I am his oldest and best friend. We were friends as kids, centuries ago." Missy replied, causing Bill to frown deeply.
"What happened?" She asked. Missy frowned.

"Why do you want to know? Awful lot of very personal questions." She snapped defensively.
"I just... I want to know him better. Besides, I find it incredibly weird that he has a woman locked in a vault, so I want to know about that, too." Bill replied. Missy sighed.
"What happened was that he ran away and I went insane. He called me a monster because that's what I am." Missy explained. "I'm here because I killed a lot of people, and, long and confusing story short, I had to be locked in a vault for a thousand years. A vault which the Doctor has to guard, as the only other remaining time lord." Missy continued.
Bill suddenly felt the urge to leave, knowing that she was sitting beside a mass murderer, but then, she knew that the Doctor had caused the loss of many lives too - even indirectly. This new life with the Doctor was one of extremely confusing morality.
Missy sensed her uneasiness. "Oh, calm your little brain down, I'm not going to kill you. The Doctor would be ever so upset." She said, rolling her eyes.
Bill nodded, not at all comforted. "Don't you get bored down here?" She asked, looking around the mostly empty room. Missy shrugged.
"I play piano." She replied.
"You're pretty good." Bill found herself saying. Missy's eyebrows raised.
"How do you know, dear?" She asked with a smirk. Bill frowned and ignored the time lady, who she knew was trying to embarrass her.
"Thanks for answering my questions." Bill said after a while, standing up to leave. A flash of an unreadable emotion crossed Missy's face, so quickly that Bill questioned whether she had even seen it.
"That's alright. I'm always happy to do things the Doctor would disapprove of." She smirked, going back to the facade she usually wore. Bill lingered at the door a little too long.
"I could... come back, if you wanted." She said hesitantly. Missy frowned, the disgusted expression returning.
"Why would I want you to?" She snapped, furious at the implication that she required company, and from a human, no less.
"No, I just meant... oh, never mind." Bill replied with a sigh, turning to leave. Just as the door was a centimetre from closing, she heard Missy's voice quietly whisper.
"Okay."