Arguably, Steven Moffat's second greatest creation for Doctor Who — after the Weeping Angels — is River Song, and this is the story in which she makes her first entrance. Of course, the character's jumbled timeline being what it is, as far as she's concerned this is her last entrance, the last hurrah. But as viewers, we didn't know that at the time.

This is also the story responsible for terrifying a generation of children, who refused to dally in the shadows any longer than they absolutely had to, and even then only after someone had thrown a chicken leg just to be sure there were no Vashta Nerada present.

Here are a few things that you should keep an eye out for the next time you watch.

(The episode is available on Amazon and iTunes.)

The original choice for the role of River Song was Kate Winslet, who had some history with Russell T Davies. One of her first roles was in a BBC teen drama called Dark Season, which he wrote.

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

Having placed the Doctor and Donna inside a library full of Vashta Nerada, Steven Moffat revealed that his plan for River Song's back story came out of necessity, rather than inspiration: "I needed a team of archeologists to walk in and immediately trust him. And there are some things the psychic paper will not cover... I thought, What if one of the archeologists knows him? And I thought that's a bit lame. But if you flip it on its head and she knows him and he doesn't know her, then it's suddenly rather electric and rather exciting... and the whole story unfolds without you particularly trying."

Alongside Bridget Jones and Monty Python, The Doctor mentions that the library holds the books of Jeffrey Archer, who will have been well known to British viewers as a former Conservative politician and hugely successful novelist.

In the girl's home inside the data core, there's a little model of Robby the Robot, from the movie Forbidden Planet.

There is also, tellingly enough, a child's drawing of a woman with blonde hair, visible behind Dr. Moon as he talks to her, and right next to it is another of a wolf.

In keeping with the rest of Season 4, there's a guest comedian in the cast. Steve Pemberton, who plays Mr. Lux, will be best known to British viewers as one third of the gothic comedy troupe The League of Gentlemen, appearing alongside Mark Gatiss and Reece Shearsmith, not only the future Second Doctor (OK, so he played Patrick Troughton in "An Adventure in Space and Time") but also Rasmussen in "Sleep No More."

https://www.youtube.com/watch?v=8IQJNwvtkmg

Harry Peacock, who plays Proper Dave, has an elder brother called Daniel who is a well known British character actor, writer and director. He's also a Doctor Who graduate, having appeared in the Seventh Doctor adventure "The Greatest Show in the Galaxy," in 1988.

There is a vaguely held tradition within Doctor Who scripts that when two Doctors meet, at some point they will end up noting a similar detail about each other, referenced with the word "snap." The Second and Sixth Doctors did it in "The Two Doctors," and so did Tenth, when he spotted the Fifth Doctor putting his glasses on in "Time Crash." In this episode, River Song says it to the Doctor when he sees her sonic screwdriver.

The Doctor claims never to land on a Sunday, because "Sundays are boring," which appears to be an attitude he picked up from his former traveling companion, Ace. In "Survival," Ace had complained that her Doctor — the Seventh — had returned her to her home on the worst possible day of the week, saying, "You had to pick a Sunday, didn't you? You bring me back to the boredom capitol of the universe, you pick the one day of the week you can't even get a decent television program."

The courtesy node that tells Donna that faces are donated from real dead people says the person who gave up that particular face was Mark Chambers. The real Mark Chambers is a professional singer. In fact, he's a counter tenor — the only singer in the history of Doctor Who to receive a screen credit for his vocal contribution. His voice can be heard in "Planet of the Ood" and most specifically during "Vale Decem," the Tenth Doctor's regeneration sequence in "The End of Time:"

https://www.youtube.com/watch?v=GZ9n-4UOwaI

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