This being Sherlock, of course, fans are expecting something huge from the finale (which some lucky individuals have already seen) and have scrutinised past episodes for clues pointing towards possible revelations.
What fans have unearthed may be cause for excitement considering it hints at the potential return of an old character.
The scene in question is taken from season two, episode three, 'The Reichenbach Fall,' and features a heated interplay between Benedict Cumberbatch's titular detective and recurring villain Moriarty (Andrew Scott).
"What is it all for?" asks Sherlock, to which Moriarty replies: "I want to solve the problem. Our problem. The final problem."
Fans are particularly excited because Sunday's finale is titled 'The Final Problem' - could the cunning villain be making a surprise last minute appearance? It would certainly be a shock considering he was killed off in season two.
Whatever happens, it's a sure bet showrunners Steven Moffat and Mark Gatiss have something up their sleeves that'll rock the fandom: it's also the title of the Arthur Conan Doyle story that sees the two characters meet their fates.
The series concludes its fourth season on BBC One at 9pm this Sunday. For the lowdown on what shows to binge once it's over, you can find our 2017 TV preview here.
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