"Listen" is the story in which the Twelfth Doctor takes a decisive step into an area he's never been before. An irrepressible monster-chaser, he kicks off this story by hypothesizing about a beast he does not know is even there.

And in the hunt for it, we find ourselves closer to his young life than we have ever been before. And back in a very familiar barn.

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

(The episode is available on iTunes and Amazon.)

Steven Moffat's original inspiration was to give the Doctor some time on his own, as Clara wasn't his constant companion, and let him come up with his own adventure. He told Science Fiction: "My impulse starting in that was just the idea, 'What does he do when he’s got nothing do?' Because he'd throw himself off a building if he thought it'd be interesting on the way down. You know, he’s fascinated by anything. And here he's with nothing to do so he just goes out and poking things with a stick until something bites it. And I think that’s quite interesting, isn't that? Sort of, there's a thrill seeker aspect."

He was also drawing on his experience of writing a Doctor Who short story called Corner of the Eye in 2007, which features a race of beings with superhuman hiding powers, called Floofs. He told Doctor Who Magazine it was a perfect opportunity to recreate the psychological horror of stories such as "Midnight": "I'm going to do a chamber piece, with no money, in the middle [of the eighth series], because I haven't done one in ages and I'd like to prove that I can actually write."

There's a throwback to the very earliest days of Doctor Who as Clara whispers to the young Doctor that "fear makes companions of us all." It's a quote from the First Doctor, who used it to comfort his granddaughter's teacher Barbara Wright, who has been transported back to the dawn of mankind by an eccentric man with a blue box in the very first Doctor Who story (known as either "100,000 BC" or "An Unearthly Child"). Ironically the term "companion" would not enter common use for the Doctor's various assistants and friends until the series was relaunched in 2005.

Clara also advises the young Doctor that "fear doesn't have to make you cruel or cowardly," which you could claim echoes the promise the Doctor told her came with his choice of name in "The Day of the Doctor." But given that she is now the first person to put that quote in the young Doctor's mind, this is a classic example of the Bootstrap Paradox — soon to be explained at the beginning of "Before the Flood" — in which ideas or events appear to have no clear provenance if they're copied backwards in time. Clara also says,"one day you're going to come back" and “do as you're told," both phrases that will later come from the First Doctor's lips.

While the Telepathic Circuits of the TARDIS haven't featured heavily in the new series, they were a more common feature of the Third Doctor's era, particularly in "Planet of the Daleks," in which a wounded Doctor sends a message to the Time Lords using his ship's mindreading powers. The TARDIS's telepathy also featured in the First Doctor story "The Edge of Destruction" as a kind of spider-sense, warning the Doctor that danger was on the way.

Just to be clear, when the Doctor is looking for "Wally" in the book in young Rupert Pink's bedroom, he is indeed referring to the Where's Waldo? series of children's books, created by Martin Handford. In the U.K. Waldo is Wally, or to be more precise — as Where's Wally? is a British creation — in the U.S. and Canada, Wally is Waldo.

We see the Doctor as a child, sleeping alone in the barn he will later return to with the Moment (in "The Day of the Doctor"). The idea that he was a lonely child has been referred to a couple of times in past Steven Moffat scripts, with the Ninth Doctor telling Nancy that, "It's never easy being the only child left out in the cold, you know," in "The Empty Child," and Madame de Pompadour commenting on his sad childhood as she links minds with the Tenth in "The Girl in the Fireplace."

The Doctor is clearly fond of his own onomatopoeic idioms. You can hear an echo of his Third incarnation in this line, in which he describes the silence at the end of time: "There's nothing to hear. There's nothing anywhere. Not a breath, not a slither, not a click or a tick. All the clocks have stopped."

Back in "Death to the Daleks" the Third Doctor described a shutdown of the TARDIS like this: "Not a click, not a tick. Nothing. The TARDIS is a living thing, thousands of instruments. Its energy sources never stop."

The unconscious Doctor awakens, shouting the words “Sontarans perverting the course of history!” with some urgency towards Orson Pink. This is also the very first thing that the Fourth Doctor blurts out at the Brigadier and Sarah just after his regeneration. It's a nod to the Third Doctor's first encounter with the potato men of Sontar, 1973's "The Time Warrior", in which a sole Sontaran called Linx crash-landed in 13th century England.

Hats are off to the coffee-toting Reg, who has the distinction of being played by Robert Goodman, one of classic Who's army of supporting talent, who often appear in the background of key scenes without saying much and rarely get a named character to play. He was a Mandrel in "Nightmare of Eden," a citizen in "Full Circle," a Gallifreyan in "Arc of Infinity," a party guest in "Enlightenment," a colonist in "Frontios," one of a ship's crew in "Resurrection of the Daleks" and an officer on the Hyperion III in "Terror of the Vervains."

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