The BBC has released the synopsis for the third and final episode of Sherlock’s third series, His Last Vow.

A case of stolen letters leads Sherlock Holmes into a long conflict with Charles Augustus Magnussen, the Napoleon of blackmail, and the one man he truly hates.

But how do you tackle a foe who knows the personal weakness of every person of importance in the Western world?

Benedict Cumberbatch returns as Sherlock Holmes, with Martin Freeman as John Watson, Mark Gatiss as Mycroft, Rupert Graves as Inspector Lestrade, Una Stubbs as Mrs Hudson, Amanda Abbington as Mary Morstan, Louise Brealey as Molly Hooper and Lars Mikkelsen as Charles Augustus Magnussen.

It airs Sunday 12 January at 8.30pm – 10pm.