Sherlock is still Sherlock, even if Holmes is just made up. He's a 5,000 year old Vampire whom has just met his Mate. Of course, John Watson has no idea and never will if Sherlock has anything to say about it.
A character whom is genderfluid (transvestite), as well as another that is pansexual feature very heavily in this story (although not at all in an explicit NC 17 sense), so if the character or the concept bothers you maybe it is best if you don't read. Just a fair warning.
This will become a crossover into Star Trek: Into Darkness, but not until the very end.