Does this new Sherlock image reveal what we can expect from series 4?

Appearing in the second Holmes novel the Sign of the Four (often retitled as The Sign of Four), said dog is called Toby and is borrowed by Holmes from a Mr Sherman to track down two villains who are attempting to steal Indian treasure from Watson’s future wife Mary Morstan (it’s quite a complicated story).

Described as “an ugly, long-haired, lop-eared creature, half spaniel and half lurcher, brown and white in colour, with a very clumsy, waddling gait,” Toby is a brilliant tracker who leads Holmes and Watson across London to eventually track down the baddies, leading Holmes to sing his praises.

As he commented in the story: "I would rather have Toby's help that that of the whole detective force in London". Now it looks like modern Sherlock will have that help again.

Toby the Dog in a 1983 TV adaptation of The Sign of Four, starring David Healey as Watson and Ian Richardson as Holmes

Now, we’re not saying that one of the new stories will be adapting the plot of the Sign of Four – last series’ The Sign of Three already used elements of the plot for John and Mary’s wedding, down to the inclusion of a character called Major Sholto – but it could be that series creators Mark Gatiss and Steven Moffat were sad not to include Toby’s iconic scenes for the episode, and found a way to include him in helping to solve a different mystery.

After all, it’s not like they haven’t pulled certain parts of classic stories into other adaptations before, with series one’s The Great Game including quotes from A Scandal in Bohemia and The Final Problem, only for those stories to get full retellings in series two’s A Scandal in Belgravia and The Reichenbach Fall.

Other Sherlock episodes have also been known to combine elements from different stories liberally, so it’s not too much of a stretch to think that this new dog is Toby finally getting his modern screen debut.

Alternatively it could even be that Moffat and Gatiss wanted another crack at The Sign of Four, using the main plot elements they skipped over last time to create a new story that will more closely adhere to the original.

But whatever the reason, it looks likely that this dog is supposed to be Toby coming to screens after a long absence, and we can’t wait to see him in action. As they say, every dog has his day – and Toby could finally have his.

Sherlock will return in the new year