The single word, "Moriarty", wrenched from a dying man in the first ever episode of Sherlock. The allusion to a shadowy figure know only as "M" in the second instalment. And the appearance of a seemingly harmless character eventually revealed to be the villain himself in the season finale.
This was our gradual introduction to Sherlock’s first arch-nemesis Moriarty in the BBC1 detective drama – and his replacement could be unveiled in a similarly slow-burning series of subtle references when the show returns for series three on New Year's Day.
Moriarty is almost certainly dead – as Steven Moffat noted “He shoots himself in the brains. You don’t come back from that”. But Moffat has revealed that "terrifying" new villain Charles Augustus Magnussen (played by The Killing star Lars Mikkelsen) will make his official debut in the season finale, and Martin Freeman, aka Sherlock's companion John Watson, says there will be "hints" about the detective's new nemesis from the beginning.
Freeman was giving little away when RadioTimes.com spoke to him earlier this year but he did tell us there would be clues laid as early as episode one.
"Obviously Sherlock, does have to have some sort of enemy, a nemesis," said Freeman. "And as we’ve seen, unless Moriarty did something incredibly Derren Brown-like when he shot himself in the head, we assume... there is no more Moriarty.
"But there will be a nemesis of some sort and the hints that we get about that in this first episode are very intriguing."
The opening episode of series three, The Empty Hearse, will see Sherlock return from the grave to tackle a terrorist threat on the London Underground. Whether it will be orchestrated by Magnussen remains to be seen but it appears there will be some clues about the as-yet-unseen villain that eagle-eyed Sherlock fans should look out for.