Since Sherlock’s (Benedict Cumberbatch) return, it seems that no one great criminal has stepped up to challenge him. But His Last Vow reveals an evil mastermind at work deep beneath the surface.

In episode two Sherlock, Season 4, Sherlock faces perhaps the most chilling enemy of his long career: the powerful and seemingly unassailable Culverton Smith, a man with a very dark secret indeed."The ...