I suspect we're going to have to start viewing Sherlock specials in the same way we view Doctor Who Christmas specials. Despite being cut from the same cloth as a regular season, there's just so much more space to muck about in. Half of tonight's episode played like a Victorian chiller, the other half was like watching panto. Still, with the festive season still in full swing, that was probably the effect they were hoping for—and it kind of worked.
