(Photo: BBC America) 
The story so far: A fireball that is impervious to dog spaceships is eating the universe. The Doctor, Dan and Yaz – and a new fellow called Vinder – are lost in time, wearing different coats and playing out alternate versions of their own pasts, but with an unnerving sense of dislocation.
“Once, Upon Time” is kind of an exposition story, in that it contains backstory information about some key characters, not least the Doctor herself. It’s also heavy on the fireball lore of time travel, so there’s a lot to delve into at first glance.
Here are a few things to bear in mind, the next time you watch:
1. As well as establishing which of the two Doctors on screen is the most recent, this story marks the official retitling of Jo Martin’s incarnation as the “Fugitive Doctor.” The term had taken root within fan discussions after her first appearance in “Fugitive of the Judoon,” but her credit for that story – and for “The Timeless Children” – was simply “The Doctor.” 

2. The Doctor uses the zen quote “leap and the net will appear,” which is attributed to the American nature writer John Burroughs. A very Doctor Who figure, Burroughs wrote essays on the natural world that chimed strongly with the readers of the late 1900s, who were busily reappraising man’s relationship with nature in the wake of, among many things, the work of Charles Darwin. He also had a fine line in thoughtful aphorisms, including, "A man can fail many times, but he isn't a failure until he begins to blame somebody else."
3. Fans of British TV drama will recognize Craig Parkinson, who plays the chilling Grand Serpent. Craig’s long and varied career includes playing twin rotters Jimmy and Johnny Kray in Whitechapel, Shaun in Misfits, DI Matthew ‘Dot’ Cottan in the BBC police corruption drama Line of Duty. And his wife is Susan Lynch, who played Angstrom in “The Ghost Monument.”
4. He also hosts The Two Shot Podcast, in which he interviews British actors and performers about their work. Even before filming “Flux,” his podcast had a surprisingly high strike rate for Doctor Who actors, including Jodie Whittaker, Mandip Gill, Ingrid Oliver (Osgood), David Morrissey and Reece Shearsmith. As of February 2022, the most recent episode stars Arthur Darvill.
5. Let’s talk video games. The arrival of a Weeping Angel in the middle of Sonya’s game (and later on Yaz’s phone) is a nod to two Doctor Who-themed games, as well as tipping a hat to the zombie apocalypse game Resident Evil.
Sonya is attempting to play a game that apes the virtual reality console game Doctor Who: The Edge of Reality…

… while the Angel in Yaz’s phone is a nod to the iOS/Android game Doctor Who: The Edge of Reality. Both games had been released in early 2021:

6. There’s a beautiful story behind the tattoo designs in the Mouri’s makeup and design. According to their official Instagram, Squarepeg Studios had been working to create tattoo designs based on numeric code. Creative director Danny Marie Elias decided to put the code to good use, as the post explains: “Hidden within these codes is a time capsule of her own. A sweet love note to her grandmother, who unfortunately was diagnosed with cancer at the time.”

7. And while we’re on hidden messages, when Bel’s story first appeared, a couple of interesting theories came up about her name. The first is that she was named for Thaddea Graham’s native County Down accent, as a shortening of Belfast. She even acknowledged the possibility in a lovely video she posted on Twitter (with an accompanying song) after the show aired, noting: “Bel's accent was never a question. I auditioned in my own accent and nobody said anything. That's something I really love about this show, everyone is accepted wherever you're from, whoever you are, whatever you are it doesn't matter. It's about being kind and it's about being curious, being generous, and I think that is the heartbeat of it, as are (Doctor Who fans).”
8. The second theory is more typical of how fandoms react to new information – and similar to those fans who see Bel’s pregnancy and immediately conclude she must be the Doctor’s mother. Twitter user DrewbooWho used a little nominative determinism to conclude that Bel is actually the Master:

9. The Doctor’s surprise on discovering that a Weeping Angel could enter the TARDIS should perhaps be set against the evidence that they’ve managed it before. The mini episode “Good as Gold,” written for the Eleventh Doctor’s TARDIS team by the pupils of Ashdene School in Cheshire, also features an invasion of the quantum-locked statues:

10. On that note, did anyone else feel the urge to reply to the Doctor’s cliffhanger comment, "The Angel has the TARDIS” with a line about merchandise? It echoes the comment in “Blink,” when the Tenth Doctor confirms to Sally Sparrow that his ship has been taken, and her friend Larry exclaims: “’The angels have the phone box.’ That's my favorite, I've got it on a T-shirt.”

Do you feel like you know this episode a little bit better now?