'Sherlock's return included so many nods to the show's fans, with red herring explanations for how he escaped certain death two years before, you could be forgiven for missing the biggest in-joke of all.
Writer Mark Gatiss pulled the biggest surprise out of his hat, when Holmes' parents turned up for a few minutes half-way through... played by Benedict Cumberbatch's own real-life parents.
Some fans went crazy on Twitter, while others were a bit confused. HuffPost would humbly like to mention that they spotted it at the time... proof below...
Oh, wow - Benedict Cumberbatch's REAL parents playing his parents. Now that's class. #Sherlock— Caroline Frost (@Frostalicus) January 1, 2014
When Cumberbatch's parents Timothy Carlton and Wanda Ventham, both actors themselves, first appeared on screen, they appeared to be two clients of Sherlock's wasting his time with a case of a missing lottery ticket. Only when Dr Watson came calling did it transpire they were, in fact, Holmes' actual parents.
As they departed with a final "You never call, you never write," Holmes aka Cumberbatch bundled them out with embarrassment, speed and awkwardness… just like a proper son.
"They're so ordinary," said Watson. "It's a cross I have to bear," said Holmes.
Cumberbatch said of his parents joining him on set, he was "so proud of them" but admitted "it was kind of nerve-wracking".
He said: "They're Equity card carrying members but you know it was nerve-wracking because they are actors as well and yet they were brilliant and they were fantastic.
"We did the Baker Street scenes quite early so I think everyone was still getting back into it."
It was a light touch, swiftly and wittily executed, and the kind of in-gag you can really only get away with if you are the acknoweledged star of a global hit, with fans hanging on every tag, reference and meta-allusion you can crowd into 90 minutes…
Gatis acknowledged the fans of the show from the start, including one of the many conspiracy theories that have been circulating since Sherlock apparently jumped to his death on the pavement at the end of Series 2.
The combination of a bungee rope, a Sherlock mask on a dead body and even Derren Brown's tools of mind-control turned out to be a red herring, as did a love affair between Holmes and Moriaty on the roof, AND Holmes stopping his pulse temporarily with a small ball. And there are 13 possible solutions apparently, according to Holmes's calculations, so we must expect a few more red herrings along the way before we're finally told the truth...