The summer after the Tri-Wizard Tournament, Harry Potter disappeared. Utterly. Twenty years later, Sherlock Holmes stumbled onto the case.

Sherlock Holmes, vampire Detective, introduces John Watson to the most improbable man in the multiverse. The world ends (a bit).

When John Watson shoots a cabbie to save the life of Sherlock Holmes, Mycroft Holmes orders that the pair of them be put on Surveillance Level 3, Active.   Things go straight downhill from there.

Spoilers for all three seasons plus a bit of speculation about what might happen after.