After that dramatic ending to episode two of Sherlock, season four, fans have been left shocked by the revelation that Sherlock and Mycroft have a secret (evil) sister.

Eurus, named after the Greek God of East Wind, was revealed to have been disguised as three different characters on screen, including as Watson’s therapist.

She also appeared as the daughter of episode two’s villain Culverton Smith, and of course, the woman Watson engages in a flirtation with.

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‘Miss me?’ – Spot Culverton on the poster in the background, with Eurus in disguise flirting with Watson (BBC One)

The third Holmes sibling, played by Sian Brooke, reveals herself to Watson as Sherlock’s sister, then points a gun at him and shoots.

Sherlock co-creator Mark Gatiss, who plays Mycroft Holmes, said at a special Bafta screening that an extra sibling had been discussed for years.

“It started as a joke years ago that there was going to be a line in the first season where we were sketching in the Holmes family. But we thought we wouldn't do much and thank god we didn’t because it has given us this place to go.

“This was a huge thing to cast Sian as Eurus. We said to casting someone who should already be a huge star who has never had the breaks.”

Steven Moffat said that Brooke had to lie to her friends about what she was doing: “She has been on TV for two weeks and now finally she is allowed to say she is in this show.”

Director Nick Hurran explained: “We disguised her by... either you shoot from above, or in any of the transitions you delay how quickly you put the two together, apart from when you want to then tell that story.”