Mary only hit on/slept with John in the first place because she's on assignment to get closer to Mycroft - her bosses notice that since Sherlock's death, John seems to be the only one Mycroft feels indebted to. (Him being responsible for handing Moriarty all the info necessary to frame Sherlock, and all.) The idea was that she date John, get John to ask something specific of Mycroft, and Mycroft will feel too guilty to say no.

Sherlock turning up - very not dead - throws a wrench in the works. He deduces her background, of course, but Mary can't figure out why he's not warning John away from her. (Maybe he believes John will be hurt and will be mad at him either way so he might as well get to be happy first?) Suddenly she has to find a way to adjust the mission, convince John and two very suspicious Holmes brothers that she "just happened to" fall in love with John (nothing to do with work), and still somehow get Mycroft to do/say whatever he was supposed to in the first place.

I do love Johnlock, but I can see this being any rating and anywhere between a single tell-all conversation to a much longer saga :-)