The actor explains: "We know from the end of series two he staged his death, he was out of the game but calculating every move and therefore in charge.
"[But] What’s exciting about this series is we see Sherlock in real peril.
"In the third [episode] they are challenged by a situation and a master villain who brings him to his knees."
He adds: "He really does lose his authority on the situation and it’s really exciting to see a hero in that much jeopardy."
So, who is this new villain? Played by Borgen and The Killing star Lars Mikkelsen, Charles Augustus Magnussen "is a businessman," says Cumberbatch.
"He’s not a mad chaotic villain, not like the award-winning performance Andrew Scott gave as Moriaty.
"He is so measured and precise – he is like a shark, a terrifyingly perfect predator of our age"
Before that, there is the small matter of Sherlock's return to deal with and it seems John Watson (Martin Freeman) isn't too happy about him faking his death.
"[Sherlock] is expecting a wry smile, a handshake and maybe a laugh or two and then off on a case. He gets it so wrong, so so wrong."
He explains: "They start from a point where they are almost strangers again."
But fear not, the duo are soon back on top form, and "they have some extraordinary adventures in the first and second episodes," promises Cumberbatch.
* Sherlock series 3 airs on Wednesday 1 January 2014 on BBC One