In "The Sontaran Stratagem" we get to introduce the Doctor to Donna Noble's family while reminding him of the devastating affect he just recently had on the family of his former traveling companion, Martha Jones. We then get to reintroduce him to an old adversary and clone Martha into the bargain.

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.)

Part of Helen Raynor's original script was that the poisonous gas would come from carbon-neutralizing chimneys that every household was having installed, rather than cars, but it felt more like an everyday threat to use the idea of the ATMOS satellite navigation system.

This episode marks Dan Starkey's first appearance as a Sontaran. Best known as Strax, of the Paternoster Gang, Dan has actually played at least five Sontarans in total, including Commander Scorr and Lieutenant Skree in this two-part story, Commander Jask in "The End of Time" and Commander Skarr (and another Sontaran solder) in "The Time of the Doctor."

In the commentary for this episode Helen Raynor explains the length the script department had to go to in order to remove the United Nations element from the acronym UNIT. The real United Nations had requested their name be disassociated from Doctor Who, while allowing the show to claim that the fictitious taskforce had U.N. funding, which meant they had to think of a new variation to keep the acronym. "I came up with two rubbish suggestions which are so bad I actually erased them from my memory," said Helen, "and then Russell came up with UNified Task Force, a semi-acronym which of course actually comes out as UIT, not UNIT, but hey, we're among friends."

Helen also revealed that she chose the names of some of her characters from members of the Doctor Who production team. For the UNIT soldier Ross Jenkins she took the first name from Ross Southard, script assistant, and the surname from Richard Jenkins, interactive producer of Doctor Who web content.

Continuing this season's theme of bringing in special guest stars from classic British TV comedies, Christopher Ryan returns to Doctor Who (after appearing as Kiv, leader of the Mentors in the Sixth Doctor adventure "Mindwarp"), as General Staal. Christopher will have been most familiar to British viewers in his more human form as Mike, the cool person, from the early '80s comedy The Young Ones:

https://www.youtube.com/watch?v=F4dt4ThrdQc

When the Doctor first meets Rattigan at his academy, he asks, "Is it PE? I wouldn't mind a kick around, I've got me daps on." Daps is a British slang term—common to Wales and the West Country—for canvas tennis shoes, of the sort worn by children for gym class. It's specifically aimed at the unpadded canvas shoe with a rubber sole—also called plimsolls–rather than the full Nike sneaker. There are several different nicknames across the British Isles for these shoes, including "gutties" or "sannies" in Scotland and Northern Ireland and "pumps" in Southern and North-Eastern England. And of course, fans of "The Day of the Doctor" will already know the term "sand shoes," which was their original British name of choice.

During filming, Catherine Tate had not initially been aware that there were people inside the Sontaran costumes. She later told the BBC TV show Friday Night with Jonathan Ross that she thought they "ran on electricity," and was entirely bewildered when one of them took his helmet off, revealing the human face of the actor beneath:

https://www.youtube.com/watch?v=dh078GkLFF0

While the scenes in the basement with the cloning tank were filmed in a Welsh shampoo factory, the actual cloning vat already had a starring role as the fountain from "The Fires of Pompeii," while the table on which Martha is held was also used in the Torchwood story "Cyberwoman."

When Donna asks the Doctor about his former role within UNIT, he replies that he used to work there a long time ago, "Back in the '70s. Or was it the '80s?" This is not just temporal scattiness on his part, some of the Doctor's UNIT adventures were set in Earth's future at the time of filming, and some were not, making a definite chronology very hard to work out. For example, the Second Doctor story "The Web of Fear" was shown in 1968, but set in 1975, while that year's broadcast of the Fourth Doctor adventure "Pyramids of Mars" suggests the Doctor met Sarah Jane Smith in 1980, when her first story on screen—"The Time Warrior"—was first shown in 1973.

The exploding ATMOS set in the UNIT Jeep was originally supposed to be a far more dramatic affair, with the car being destroyed in a fireball, in classic action movie style. However, budgetary constraints demanded a crafty script re-write, downgrading the explosion to a few sparks, which could then be used to poke fun at that kind of action movie stunt. That's called having your cake and eating it.

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