The campaign began last night with a series of tweets from Sherlock co-creator Mark Gatiss – "It's all gone dark... Something's coming... Or someone..." – during which BBC1's official Twitter account weighed in with an image of Sherlock's arch enemy Jim Moriarty and the now familiar question "Did you miss me?", along with the hashtag #221back.
It had the desired effect. Excitement spread like wildfire through the ranks of the Sherlock faithful, and speculation as to what it all meant went into a frenzy.
Well now we know – the detective will return for a one-off special followed by a new three-part series, with filming set to begin in January 2015.
The stand-alone episode will tackle the mystery the detective was faced with in the final frames of series three last January – why the face of Sherlock's supposedly dead arch-enemy Moriarty is being broadcast on every television in the land – while co-creator Steven Moffat says "we're reasonably confident that the very next thing to happen to Sherlock and John, is the very last thing you'd expect...”
The announcement also appears to back up claims by Martin Freeman – aka Dr John Watson – who said recently that fans could expect a Christmas special in 2015.