"The Power of Three" is a story about growing up and risk. It's about choosing when to be responsible and when to chase adventure, and it puts an impossible choice before Amy and Rory Pond—to pick domestic bliss or their lives roaming time and space with the Doctor.

As we will come to see, their choice—aided by Rory's father Brian—will come to have dramatic consequences for everyone involved, but this is a last hurrah for the family that grew around the Eleventh Doctor from the minute he crashed into young Amelia Pond's backyard.

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

(The episode is available on iTunes and Amazon.)

Chris Chibnall was originally supposed to write a story for the Doctor and his new companion later in Season 6, after he and the Ponds parted company, but Steven Moffat liked his handling of the domestic interplay between the characters in "Dinosaurs on a Spaceship" he asked for a third script, which was originally entitled "Cubed." Shortly afterwards, ITV commissioned Chris's first season of Broadchurch, so he had to turn down the offer of the third script.

The inspiration for the story was the 2007 shipwreck of the MSC Napoli, a container ship that foundered off the coast of Devon. There was a wealth of goods aboard ship at the time, and this caused something of a frenzy as they wound up on local beaches. Chris liked the idea of people chasing after free things without knowing their true provenance.

https://www.youtube.com/watch?v=7eRiJCB9CjE

The phrase "power of three" is a pun, using the mathematical meaning of power to suggest that a combination has greater resources than the sum of its parts. It's been used to denote the Holy Trinity, the combination of body, mind and soul, and the combined powers of Prue, Piper and Phoebe (or more recently Piper, Phoebe and Paige) in the TV series Charmed. As a phrase, it was used as the title of a fantasy children's novel by Diana Wynne Jones, and the title of a 1996 episode of Teenage Mutant Hero Turtles. It even has a Celtic symbol, three connected circles.

Chris Chibnall wanted to pay tribute to Nicholas Courtney—the actor who played the Doctor's old friend the Brigadier—who had died in February 2011, and bring back something of the Earthbound era of the Third Doctor in this story. But rather than have the Doctor sparring with another military figure of authority, he created Kate Stewart to embody a mixture of science and military force. The Brigadier had already been bequeathed a daughter called Kate in the made-for-video production "Downtime," but Chris did not realize this when he wrote the script.

There's a neat, if sad, Easter egg in this story. One of Amy and Rory's 59 phone messages concerns some reading glasses which are ready for collection. These are the glasses Amy is wearing while reading in Central Park in "The Angels Take Manhattan," and the same glasses the Doctor uses until his regeneration. He can be seen wearing them as an old man in "Time of the Doctor."

At the time the Doctor tells Brian that "some" of his previous companions have died, there are only two who have traveled with him in the TARDIS and have passed on as a result (in the TV versions of his various adventures, at any rate). The First Doctor befriended a citizen of ancient Troy called Katarina, who was killed in her very first adventure ("The Daleks' Master Plan"). During the same encounter, a security officer called Sara Kingdom took on a lot of what we would think of as companion roles, but she also died at the hands of the Doctor's oldest foes, and is sometimes considered not to be a full companion, for the same sort of reasons that Astrid Peth isn't, despite being invited to travel with the Tenth Doctor in "Voyage of the Damned".

Adric, traveling companion of the Fourth and Fifth Doctors, died while trying to decode the locked navigation controls on a spaceship headed for prehistoric Earth. Since the events in this story, Clara Oswald met her end in "Face the Raven."

https://www.youtube.com/watch?v=Hn2W9vn2yjs

There's a cameo from Sir Alan Sugar, from the set of the UK version of The Apprentice. The reality TV show was filming at the time, so director Douglas Mackinnon stood in for the hapless contestant who was fired on camera.

Brian Williams describes himself as being "cream crackered" while watching the cubes. This is cockney rhyming slang for the term "knackered," which means tired. The latter term derives from "knacker," the job title of anyone who disposes of animals which are unfit for human consumption, so if you're knackered, you're exhausted beyond all use.

While playing Wii Tennis, the Doctor says "Oh, if Fred Perry could see me now, eh? He'd probably ask for his shorts back," a reference to the British tennis and table tennis player. Perry was a World No. 1 who won 10 Majors including eight Grand Slams and two Pro Slams single titles, as well as six Major doubles titles. He also won three consecutive Wimbledon Championships from 1934 to 1936 and was World Amateur number one tennis player during those three years.

As has become traditional whenever Doctor Who wishes to show typical public scenes of a British Christmas, the song heard in the background when Rory is at work in the hospital at Christmas is "Merry Xmas Everyone" by Slade. It was heard in Mickey Smith's workshop in "The Christmas Invasion," it was played at Donna Noble's wedding reception in "The Runaway Bride" (and in the alternate Christmas she experienced in "Turn Left"). It was heard in electronic form when Donna's mother opened a Christmas card in "The End of Time" and it was also the song Shona McCullough played to stop thinking about the dream crabs in "Last Christmas".

https://www.youtube.com/watch?v=apoFZv5J6xo

NEXT: "The Angels Take Manhattan"

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