Fan mashups are usually all about the editing, blending together of different scenes from different episodes/films of different franchises in order to give the impression they are part of the same story.
"Wholock" goes far beyond that. VFX whiz John Smith has used various bits of digital trickery to blend together Benedict Cumberbatch's Sherlock Holmes and Matt Smith's The Doctor in a five minute short. With the exception of a couple of scenes within the TARDIS, for the most part it's surprisingly seamless and extraordinarily good production value wise.
There's also an accompanying effects breakdown video to showcase how Smith pulled off this bit of digital wizardry.