"The Impossible Planet" is a classic type of Doctor Who story—the siege—in which all the characters are trapped on a base or space station, trying to fend off an attack from without that no one fully understands. Oh and the TARDIS is unavailable, because that would be the easiest solution for everyone involved.

It is also the story in which we discover the wonderful Ood, in both their servile and furious forms.

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

(The episode is available on iTunes and Amazon.)

Russell T Davies came upon the idea for this two-part story by wanting to challenge the Doctor's beliefs in how the universe works, which also happened to neatly tie in a comment by series producer Phil Collinson that the Doctor hadn't yet (in the new series at least) met up with a sole adversary that was godlike in size.

He brought in Matthew Jones, who as well as writing for Russell's series Queer As Folk, had penned the Doctor Who: The New Adventures novel "Bad Therapy" in 1996. His original intention was that the entire story would be called "The Satan Pit," which became the title of part two of the story, and that the planet circling the black hole would be called Hell.

Other subtle Satanic references include the Doctor claiming that, in order to generate the gravitational field that keeps the planet circling a black hole, "you'd need a power source with an inverted self extrapolating reflex of six to the power of six every six seconds." And both of the episodes in this story were broadcast on either side of June 6, 2006 (or, to give it it's numerical title: 06/06/06).

Another early idea was that the slave race the Doctor and Rose encounter would actually be Raxacoricofallapatorians, who would pray to the beast as a god to rescue them from captivity. The name Ood was chosen by Russell to be a play on the word odd.

The quiet demeanor and immaculate manners of the Ood are deliberately reminiscent of the Sensorites, as met by the First Doctor (in "The Sensorites") under remarkably similar circumstances. They appear to be threatening at first, then reveal their gentle motivation. Russell T Davies even claimed (in Doctor Who Confidential) that he believed the Ood planet to exist somewhere near the Sense Sphere, home of the Sensorites:

https://www.youtube.com/watch?v=0P-aj6PB6S4

"The Impossible Planet" hosts the debut appearance of what is now an iconic outfit for Doctor Who, the orange spacesuit. It will reappear on the Tenth Doctor shortly in "42," then again in "Waters of Mars," in which he damaged the helmet. The Eleventh Doctor has clearly either fixed it or happened upon a new one when he travels back to the Earth's earliest days in "Hide," and by the time the Twelfth Doctor wants to take a walk on the surface of the moon ("Kill the Moon") he's managed to amass enough of them to offer spacesuits to both Clara Oswald and Courtney Woods.

The nightshift's "chosen track for transition" is Ravel's Bolero, a piece of orchestral music that remains best known to British viewers (or at least, those without a particular affinity for symphonic works) as the soundtrack to this 1984 Olympic gold medal-winning ice dance performance from Torvill and Dean:

https://www.youtube.com/watch?v=t2zbbN4OL98

The Beast is voiced by Gabriel Woolf, another Doctor Who veteran who had previously lent his hissing evil tones to the Egyptian rotter Sutekh the Destroyer in the Fourth Doctor adventure "Pyramids of Mars." This isn't the only thing the Beast and Sutekh have in common. The Ood state that the Beast has many names—"Some may call him Abaddon. Some may call him Kroptor. Some may call him Satan or Lucifer"—and the Fourth Doctor made a similar claim of Sutekh while turning down an offer of an alliance: "Serve you, Sutekh? Your name is abominated in every civilized world, whether that name be Set, Satan, Sodos."

https://www.youtube.com/watch?v=V_VY4mbR7lo

Gabriel's voice wasn't added until post-production, and while the rest of the episode was being shot, a story appeared in the tabloid press that the Doctor Who production team was considering enlisting the services of Chris Evans to voice the Beast. Not the Chris Evans who plays Captain America, the Chris Evans who now hosts Top Gear and was at the time married to Billie Piper.

The Doctor has a moment with Ida where he mocks her for tempting fate by saying, "There's no turning back." "Oh did you have to?" he quips, "That's almost as bad as 'Nothing can possible go wrong,' or 'This is going to be the best Christmas Walford's ever had.'" That second one is a reference to the BBC's long-running soap EastEnders and their Christmas specials, which always begin with the brightest of intentions and then collapse into fighting, recrimination, infidelity, shouting and general unremitting misery.

NEXT: 10 Things You May Not Know About ‘The Satan Pit’

Now read the rest of the 10 Things You May Not Know About Doctor Who archive.