In the end, Missy and Master decided to grab their coats (both black) and scarfs (his - green, hers - red), go out to buy groceries (even if it was that awful time of the year) and pray that shopping queues won't be that long. (Well, they were hungry.)
"Oh, that's nice." Master said, when they both were coming back. "You put a mistletoe on the top of Tardis."
"Huh?" Missy stopped admiring a silver ring, which she was given from a rich naughty old man, who planned to buy it for his young girlfriend. The time lady mercifully told him that she won't inform his wife about it. "No, it wasn't me."
"You were the last to exit it." Master said from between two huge shopping bags in his hands.
"It doesn't mean that it was me."
"What? Then who?"
"You, obviously." Missy answered and made sure that the content of her bag won't fall out. "I've been with you all the time."
"When? I was with you as well. (... and I had to drag you out from that shopping mall with hats.)"
"Then It was probably some kid/salesman/Santa Claus. Unfortunately, it's Christmas. People start to get crazy ideas and hang mistletoes, wherever it's possible."
"Our Tardis changed into a huge wooden box in some dark corner. I don't think that it's an appreciate place for a mistletoe."
"Then it was done by a nuts kid/salesman/Santa Claus. Can we get inside? It's cold here."
"You know, Missy. It's just right above the entrance." he smirked.
"Yeah?"
"Do I get a kiss?"
ooo
Of course, it was the Master. In two more days, he would travel back in time to that exact moment to put a mistletoe on the top of Tardis. He just hoped that Missy would never find out.
Of course, she would.