"The Snowmen" has a tough job to do, to make festive fun out of a heartbroken Doctor who has given up. It shows our have-a-go hero acting against his own natural instincts to get involved whenever strange events threaten his immediate environment, and to befriend the sparkiest, most inquisitive person he comes across.

He starts the story entrenched in sorrow and quite bitter, and ends it back to his old self and with a new mystery to solve: Clara Oswald, the impossible girl.

Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on iTunes and Amazon.)

As a taster for this Victorian epic and a clue as to the true depth of the Doctor's despair, the Doctor Who production team trailed the story with three minisodes. The first is called "The Great Detective", and was shown as part of the BBC's Children in Need charity telethon in 2012:

https://www.youtube.com/watch?v=G17_B4uACgg

This was followed by "Vastra Investigates," which sets up the story:

https://www.youtube.com/watch?v=Rirju6is4Sw

And the third, only available in the US on iTunes and Amazon, is "The Battle of Demon's Run — Two Days Later," which explains how Strax came to be in Victorian London, given that he was last seen dead in "A Good Man Goes to War."

The idea of showing the Doctor feeling morose and effectively turning his back on the interfering and traveling that are the hallmarks of his character is not a new one. Even as far back as 1979 script editor Douglas Adams suggested a crisis of confidence for the Fourth Doctor as a good place to start a story from. Graham Williams, the show's producer at the time, thought the idea of the Doctor announcing his retirement could alienate the show's younger viewers.

This story was first intended to be the origin tale for the Doctor's new companion, a Victorian governess called Beryl Montague. The Second Doctor had traveled with a companion from the same era—fittingly named Victoria—but the production team were concerned that it would cause a lot of scripting headaches to have her so far behind the times, compared to her audience. It would slow down the already tight 45 minutes of storytelling if the Doctor had to keep explaining relatively recent technological advances to her. As it turned out, making "Beryl" one of Clara Oswald's alter egos would have a greater impact on future stories and her place in the Doctor's life.

The Doctor employs a very British term for women while denying that he would be thinking of helping Clara: "Do you think I'm going to start investigating just because some bird smiles at me?" The interesting thing is that the use of the term bird to mean woman has only been dated as far back as 1915, 60 years after the events depicted in this story. But, medieval Brits in the 13th century also used bird as a slang term for young woman, probably as a pun on "burde," the middle English word for girl. So only a time traveler would have used that word in the 1800s, either one coming from the past or back from the future.

Richard E. Grant, who plays Dr. Simeon, has form when it comes to playing Doctors in Doctor Who. In fact he's played THE Doctor on two previous occasions. First in Doctor Who and the Curse of Fatal Death, the Comic Relief spoof which was Steven Moffat's first Doctor Who script for TV, and secondly as a non-canonical Ninth Doctor in the 2003 BBC web animation "Scream of the Shalka". This appeared during the gap between the classic series and Russell T Davies' revived version, starring Christopher Eccleston.

According to director Saul Metzstein, Steven Moffat has three notes to offer any director on how best to approach filming an episode of Doctor Who: "One is that there is no cynicism in Doctor Who, so you shoot it like it’s a drama. The second is that you shoot it so fast that no one works out that the plot doesn’t make any sense – time travel will never make any sense. And the third thing he said was, it’s made for eight to 12-year-olds or adults who watch like eight to 12-year-olds."

If you're wondering where you may have seen Ellie Darcey-Alden—who plays Francesca Latimer alongside her brother Joseph (Digby Latimer)—before, she appeared in Harry Potter and the Deathly Hallows: Part 2, as Harry's mother Lily, when she first met Severus Snape.

The Doctor appears in a Punch and Judy puppet theater, holding the puppet of Mr Punch and yelling his catchphrase "that's the way to do it!" Derived from the Italian commedia dell'arte, the Punch and Judy show was a hugely popular comic grotesque in which Mr Punch (originally Pulcinella or Punchinello), a clownish rogue with a hunched back, battles with various authority figures, including a policeman, a crocodile, and his wife Judy, who has left their baby in her husband's care. Although Punch is shown as violent and an irresponsible prankster, whose other catchphrase is "naughty naughty naughty," he always has the sympathy of his audience, young and old alike.

https://www.youtube.com/watch?v=DheNfmrj89k

It's not often that the Doctor sets his former self up for a battle. But when the Eleventh Doctor shows the Great Intelligence the lunchbox with the 1967 map of the London Underground on it, saying the tunnels are "key strategic weakness in metropolitan living," he unwittingly (or wittingly, it's hard to be sure) sets events in motion that will lead to the Yeti attacking London via those same tunnels in 1967's eerie Second Doctor adventure "The Web of Fear". In that story and "The Abominable Snowmen," which precedes it, the Great Intelligence inhabits a small army of robot Yetis, and seemed to be well aware of who the Doctor is when they first met. The events in this story may well explain why.

There are numerous nods to Sherlock Holmes throughout this episode, with Dr. Simeon mentioning "Doctor Doyle" and his stories in The Strand magazine (and claiming the tales are based on the adventures of Madame Vastra); the Doctor dressing in Sherlock's trademark deerstalker hat and tweed cape and Murray Gold's parody version of the Sherlock TV theme. But did you know that Matt Smith had auditioned for the role of John Watson, losing out to Martin Freeman despite Martin's claims that his first audition had been a total disaster?

Also, this isn't the first time the Doctor has cosplayed as Sherlock Holmes. The Fourth Doctor did the same thing when visiting London a few years after the events in this story ("The Talons of Weng-Chiang").

https://www.youtube.com/watch?v=77cgmvR2Pas

NEXT: "The Bells of Saint John"

Now go back and read the entire 10 Things You May Not Know About Doctor Who archive.