"The Poison Sky" is not the first Doctor Who story in which Catherine Tate, Billie Piper and Freema Agyeman make an appearance—that honor is spread across the two episodes of "Army of Ghosts" / "Doomsday"—but it is the first single episode in which Donna Noble, Rose Tyler and Martha Jones all appear. It's also the dramatic conclusion to the ecological thriller that returned the Sontarans to the modern series.

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.)

Unit personnel use the radio call signs "greyhound" and "trap," followed by a number—as illustrated in the Doctor's pained protestation after Ross Jenkins's death that "He wasn't Greyhound Forty. His name was Ross." This follows a tradition established in all of the UNIT stories during the Third Doctor's era.

There was a plan by this episode's director Douglas Mackinnon to speed up the TARDIS console's characteristic rise and fall movement during the story's dramatic peak. But the set was damaged in the attempt, and the idea was quickly abandoned after filming had to stop to make repairs.

The Doctor's insolent skipping of the Sontaran transmission brings up the BBC children's ecological superhero cartoon Tommy Zoom on screen. It's by no means the first time that children's TV shows have made their way onto Doctor Who (see previous 10 Things You May Not Know About Doctor Who posts for details). And despite the aptness of Tommy Zoom's ecological message in a story about global poisoning, the show was actually second choice, after a deal fell through to put Aardman Animations' Shaun the Sheep on the screen instead.

After Donna whacks the Sontaran guard with the TARDIS's mallet, she says "Back of the neck!" in a satisfied tone of voice which is almost entirely borrowed from the British comedy creation Alan Partridge (as played by Steve Coogan):

https://www.youtube.com/watch?v=kEl5RvbGdik

Donna's inability to pronounce the word "Sontaran" correctly is thought to be a nod to a conversation that took place during the production of "The Time Warrior," the Third Doctor adventure that first introduced the potato-faced warrior race. Director Alan Bromly queried the pronunciation of the word by actor Kevin Lindsay (who played the Sontaran Linx), insisting it should be "SON-taran," rather than "Son-TAR-an." Kevin is said to have replied: "I'm from the bloody planet, I think I know how to pronounce my own name!"

In the scene set in New York, there's a building with a "Butler Institute" nameplate on it. This is a reference to a corporation investigated by the Seventh Doctor in The New Doctor Who Adventures book by Andrew Cartmel called Cat's Cradle: Warhead. The Butler Institute sought to avoid an ecological disaster by uploading human consciousness into computers.

Helen Raynor had originally given Luke Rattigan the surname Marlow, naming him after a young Doctor Who fan she knew. Russell T Davies preferred the surname Rattigan, which had originally been earmarked for Miss Foster, in "Partners in Crime."

The Doctor's reference to the Rutan-Sontaran War is a passing nod to a long-established conflict that appears to have lasted for millions of years between two proud warrior races. Actual battles are seldom shown (the Rutans, a race of green jellyfish from Ruta 3 with the ability to shape-shift and impersonate creatures they have killed, have only appeared once on screen, in "The Horror of Fang Rock") but the bitter struggle between the two species is part of the back story for a good deal of the Doctor's encounters with the Sontarans, including "The Time Warrior," "The Horror of Fang Rock," "The Two Doctors," and "The Invasion of Time."

This story contains the first reference of the new series to Brigadier Alistair Lethbridge-Stewart, commanding office of UNIT during the Third Doctor's tenure as chief science officer. The Brigadier first met the Doctor in his second incarnation, during the Yeti invasion in "The Web of Fear," and was a witness to his third regeneration, dryly remarking "here we go again" as the bumptious Fourth Doctor arrived:

https://www.youtube.com/watch?v=snl6LqFrr1A

Apart from the various Clara Oswalds in the Doctor's timeline (from "The Name of the Doctor"), the Brigadier is the human who has met the greatest number of the Doctor's various incarnations, having encountered the First Doctor in "The Five Doctors," the Fifth Doctor in "Mawdryn Undead" and the Seventh Doctor in "Battlefield." He was last seen in Cyber form, saluting the Twelfth Doctor in "Death in Heaven."

The fleeting shot of Rose Tyler in the TARDIS screen was not part of the original script. It was only added in post-production when it became clear that her unannounced appearance in "Partners in Crime" had caused a huge stir among fans. The footage used had been shot for Rose's similar appearance in "Midnight," and used to bridge the gap between the two stories.

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