He does not understand her – but he understands her completely. That's the paradox he faces every time she flickers into his life, like a candle tugged into the dark by a strong wind only to flare straight back, brighter than before.
Usually it's a roller-coaster. She is the definition of chaos and wants, more than anything, for him to chase her across time. He leaves the phone ringing when he thinks it's her. He pretends he doesn't see her fragments of thought on his psychic paper. He closes the door when the TARDIS drops him near her wild parties. He tries – oh he tries – to stay away.
It's not because he doesn't care.
Not at all.
It's because he cares far too much and he has this throat-clutching terror that they are going to destroy each other one day. Perhaps they already had.
The Doctor lingered outside the vault. Another monstrosity built by the Timelords… Confession Dials, Pocket Watches, Vaults – what was it with their pension for trapping people in time-locked torment? He'd always wondered where Missy found her pension for bondage when the answer was startlingly simple. Timelords.
He wasn't even supposed to be down here. Nardole wouldn't approve – he never did. 'A risk, you are – on your own.' On his own with Missy. Well… Tough. They'd never needed a chaperone before and only a few civilisations got caught in the cross-fire.
Strange. He can't hear the soft thrum of piano keys. The Doctor presses both hands against the cool surface – then his ear. The vault hums but it's an electric quiver not a psychopath's song.
He clenches his fist briefly with a flare of pain before typing in the codes. The vault hisses open. He wastes no time closing it behind him. It wasn't above her dignity to surprise him at the door and make a dash for it. The last time she'd done that he had to fish her out of the university fountain.
This time it's different. He sees her at once – a crumpled heap atop the grand piano. The Doctor's shoulders drop. His lungs drain of air. It hurts him, right to the marrow, to see his friend like this.
Quietly, he crosses the room and lingers outside the forcefield. It's all for show. A harmless trick of the light to quiet Nardole's concerns. They play up the lie. Pretending to snap their hands away from the forcefield in pain. She snarls at it. He broods at it with his eyebrows. Nardole won't got near it.
"Oh Missy..." He breathes, stepping up to the piano.
She doesn't quiet fit on the polished surface. One arm dangles over the side. She's using her forearm as a pillow with an endless vision of her wild hair obscuring her face. A legs hangs off the other side. Her boots are on the floor underneath and she's left wearing purple socks with silver stars.
His hearts falter – but that's because they're dying.
He can't leave her like that.
Carefully, the Doctor slips one of his arms underneath her. She shifts, curling up towards him on instinct. He's not sure if she's pretending to be asleep for a bit of attention – even so, he'd still give her this moment.
He starts to doubt a rouse when her head lulls against his chest. Normally she's broken the act by now but Missy remains a dead weight in his arms as he carriers her down the steps and across the room. He lays her on the bed, letting her slide onto the red silk.
Should he stay? He doesn't know. It's a question the Doctor considers as he attempts to brush some of her wild hair off her face.