Sherlock's Shocking Secrets

Steven Moffat and Mark Gatiss (left) on why they’ve given Sherlock an evil sister – and what’s next for the hit detective show

- Emma Bullimore

Well, we didn’t see that coming! after mary Watson’s shock death in episode one, Sherlock stunned the nation again last week, as we met a despicable new member of the Holmes clan – evil sister Eurus, who also features in this week’s series finale.

The revelation comes after much speculation about a brother for siblings Sherlock and Mycroft, with many fans hoping Benedict Cumberbatch’s friend, The Night Manager star Tom Hiddleston, would take the role.

‘We introduced Sherlock’s mum and dad a while ago. Now we’ve just taken a few more steps along that line,’ says Steven Moffat, who co-created the series with Mycroft actor Mark Gatiss. ‘People had suggested a brother, but we thought maybe we could have a sister…’

Eurus represents the dark side of Sherlock, which is usually suppressed by the warmth of John’s friendship. ‘We often think about what Sherlock would be like if he hadn’t been rescued by Watson. How would that have ended up?’ asks Steven. ‘But we didn’t introduce Eurus for a deep, thematic reason, we’re just trying to have fun with proper surprises.’

In last week’s episode, it was shockingly revealed that three characters – Watson’s therapist, Watson’s love interest and Sherlock’s late-night visitor – were in fact all the same person, Eurus, played by Sian Brooke.

‘The characters don’t look alike,’ says Steven. ‘I stuck in a couple of distractions that kept drawing your eye away, like the walking cane.