For Christmas their first year together, (Tracy) she gives Mycroft a land mine.
He loves it. He keeps it on the mantle piece until his landlord makes him remove it. To him, it is the best Christmas present ever.
The next Christmas, Mycroft gives her a cat.
She's allergic to cats.
He knows this.
It's all part of what makes Christmas fun.
The next Christmas is much, much stranger. Mycroft wakes up to find the entire ceiling throughout his house covered in mistletoe, and Bailey (her) stands there, smug as ever, and waits to be kissed.
He isn't one to disappoint.