Walking back into the vault after spending months with the Doctor was much harder than Missy expected. After he's gone and the doors are closed she remembers exactly how barren this bloody box is. It's meant to be a punishment. She knows that. Six steel walls and a thousand years with herself for company.

Solitude is the furthest thing from her mind. Oddly, she's never had more company in her life because the Doctor can't help himself. He kept popping in, every chance he got. When he vanishes for a week or two she panics – not about the loneliness – about him. He's gone and gotten himself into trouble somewhere and she's not free to help him. The Doctor's put his backup plan in a vault. Like an idiot.

Knock. Knock.

Missy frowned. He didn't knock. He just kind of – entered unannounced whenever he felt like it. When Nardole and the companion-snack entered instead Missy perked up like a cat with a pair of clueless mice. They wanted her help with the monks. The boringly easy monks.

A little later, her young Doctor is back with a coat that's seen better days. It's all faded, frayed and torn.

"A wardrobe the size of the Thames and you insist on wearing that old thing..." She playfully snarls.

The Doctor's trying hard not to blush while the others watch on – silent questions hanging in the air everywhere like swords. "Focus, Missy. Monks. Save Earth. Plan?"

She fashions a list of ridiculous demands first. The worst that can happen is that she gets a pony out of it.

When it's all said and done the Doctor comes and sits in the vault, staring at her while she plays the piano.

The Doctor has worked out what unsettles him about the vault. It's not lived in. He was gone for six months and the books are still in the same place he left them, the cups are on the sink from their last morning tea and the chairs are exactly as before. Something's going on but he can't work out what so he stares at her while she plays.

"You look different..." He started, quietly.

"Changed my hair." She replied, without looking up. This part of the song was difficult and she'd not spent as much time practising as she should.

"Six months on your own. That's a long time." He continued. "I'm sorry."

Her fingers took a misstep and the song was ruined. "Not your fault." Missy swivelled around on her chair. The Doctor was occupying the large, black seat – well he'd collapsed into it. He looked dreadful. Exhausted. "You were a prisoner of the monks. Besides, I don't mind being alone. I spent ten years in a cave once."

"Why'd you do that?" His eyebrows went in different directions.

"I was searching for the meaning of life."

The Doctor scoffed and a moment later they were both laughing. She lied as easily as he did – usually for comic effect – or to hide the damage.

"You don't look well," Missy continued, still perched on the piano stool. She could come down and join him any time she liked but today she sensed that he needed space. If she pounced he might break. "Did those monks forget to feed you?"

"No. Well – yes but that's not why." He pinched a twitching nerve in the centre of his forehead then ran his hand back through his hair. It had grown quite a bit and was as unruly as ever. "Bill – my companion – you've met her a few times," the Doctor added, just in case she hadn't been paying attention to his friends. "She sacrificed herself to save the world and it was only a fluke of the universe that she survived the act. Why do they keep doing that? Humans. They're so reckless with their lives."

"You're reckless," Missy pointed out, more serious than usual.

"I have lives to burn."

"Mmm..." she very nearly purred – in thought rather than seduction. "But you're their teacher and they learn. Humans. Race of sponges."

She was right. It was one of her more vexing properties. "I wish they'd be more like you – careful, considered. You come off as chaos and noise but all of it orchestrated. You think I don't notice but I do."

Now she was smiling fondly. "Well maybe I'm running out of lives."

The Doctor's hearts faltered. He hadn't thought to ask… "Regeneration?" He asked.

"Now that would be telling…" Missy spun around on the seat and started playing again, only this time it was a sad song.

"No adventures today, Doctor?" Missy asked, when he strolled into the vault. "The other you is off gallivanting about on Mars with the snacks so I thought we could go too. Of course if we go I want to visit before the ice caps evaporate. That purple sea is quite a sight and the floating islands… Oh I love those. I'll get my hat..."

"Not today, Missy," the Doctor held up his hand to stop her fussing.

"Why ever not? You always take me out and-" Missy noticed that he was nearly crying – eyes shining like oceans at night. "How long have you been gone..."

"A while." He replied darkly.

Missy stalked towards him, her boots clicking over the marble floor. She stopped when there was a few feet remaining between them. "Answer the question properly. I'm not one of your pets."

"Twenty years."

"Whaaaat!" She hissed, both shocked, angered and deeply concerned. "Why ever did you do that? Did you find a new playmate..."

He shook his head. "Nothing like that."

Missy was on the verge of ranting at him when she caught a different look cross that face of his. It was the eyebrows. They gave away his secrets without him even knowing. "Oh… You were..." He was trying to find a way to change the future. Trying to save her without tearing reality apart. "You mustn't do that, Doctor." Missy finished, very softly. She reached forward and placed her hand tenderly on his chest. His hearts were still there, beating. "You really mustn't."

A pair of tears fell from his eyes, so heavy they skipped over his cheeks and landed on his lapels. Missy brushed them off the fabric as though they were rain.

"You listen to me, Doctor," she fussed with his weary suit. "Whatever the universe has in store for me will come to pass and you must let it. The death of a Timelord is as fixed as fixed points get..."

He tried to stop her speaking. "Missy – you're not going to-"

"I see it – in your eyes. That's why you're sulking around in my vault – eking every last breath out of me. I watch you walk through that door – past and present – young and old – and the two of you are merging together. It can't be long now."

He wasn't even able to speak. Instead he stood there while she straightened his collar.

"So – why don't we go to Mars?"

"I wish we could but on this occasion you have to stay in the vault and I can't be here. That is just how it is."

"I have a question."

"All right..." The Doctor replied, trying to compose himself. It was difficult knowing that this was the last time he'd look at her. He was trying to transcribe her into his memory. A perfect copy.

"Why do you always leave the vault door open? Are you hoping that I'll run away? If I run away, Doctor, we never do this. Those adventures evaporate. The stars unravel."

It was only then that the Doctor realised that this indulgence of his was the thing trapping Missy in the vault – almost luring her to her fate. If he'd been cruel then maybe she'd be alive and it killed him.

He was crying again and Missy found it hard to look at him. "You always make me close the door. If that's your way of giving me a choice you've thoroughly misunderstood the definition of choice." Missy moved closer, leaning against him now with her hands on his shoulders. He did nothing to stop her. "How could I ever leave when it's not the door that's keeping me here?"

"Missy no..." He begged. He didn't want to hear that.

Missy knew that he was never coming back – not this Doctor. "I don't want to say goodbye." She barely breathed her reply. Both his hands cupped her face – more tender than she'd ever felt. How was he so warm when ice clung to her soul?

"Then don't." The Doctor replied. He found him moving toward her – drawn by the invisible threads that seemed to bind their lives together. At first, he simply rested his cheek against hers and their tears blended together. She shuddered, her resolve faltering but he caught her with a quick turn of his head – distracting her with a kiss.

This was something they did now. Missy was sure their kisses could topple worlds. They were all consuming as four hearts fought to find a rhythm. He was saying goodbye. She could feel it in her soul – see ghostly images in her mind of their lives filtering through their telepathic link. He'd locked her out for so long but there were cracks in his resolve.

"What if I can't do it?" She asked, as they both drew back to breathe. "I want to live..."

The Doctor could not even speak.