Doctor Who/Recap/S30/E04 The Sontaran Stratagem

The Doctor: Martha, Donna. Donna, Martha. Please don't fight. I can't bear fighting.

Paging the Doctor! When Martha gave the Doctor her old phone, this is the kind of situation she had in mind.

Since he left, remarkable strides have been made in environmental engineering: in particular Atmos, a gizmo that is installed in half the cars on Earth which neutralizes exhaust, with only the tiny side detail of being completely evil and a front for alien murders. Atmos is the creation of a teenage genius named Luke Rattigan, who is secretly working for the Sontarans, a warrior race of militaristic short guys who look a bit like potatoes. (Though the Doctor would like to remind you that to a Sontaran, you look like a shaved pink weasel-thing.)

Martha and Donna are introduced and, to the Doctor's great relief, don't dislike each other. Martha's now a proper doctor, and with UNIT. The Doctor's a little bit uncomfortable with how well she fits into a military organisation. She retorts that he's the one who got her the job in the first place. He wisely doesn't respond to that.

The Doctor teams up with the newly militaristic UNIT, but he doesn't like the way things have changed since the old days, and the reunion is an uneasy one. The Brigadier (now a Sir) is a few continents away and unable to help out. Also, Martha is kidnapped by the Sontarans, and they create an evil clone of her.

Meanwhile, Donna goes back home to get another earful from mom, and to see Wilfred, her dear old gramps, again. The Doctor turns up to fetch her (and meets a delighted Wilfred), just in time for all the Atmos-equipped cars in the world to begin emitting clouds of poison gas! How can the Doctor stop an alien attack that's occurring in 400 million places at once?

Tropes

Absence of Evidence: "The first thing I noticed is an empty file."

Doctor: Why, what's inside it? Or, what's not inside it?"
Donna: Sick days. There aren't any. Hundreds of people working here and no one's sick. Not one hangover, man flu, sneaky little shopping trip, nothing. Not ever! They don't get ill.

And Starring: Freema Agyeman is credited with an "And".
Authority Equals Asskicking: Colonel Mace.
Big Damn Gunship: The USS Valiant, making a return appearance, accompanied by the necessary Rousing Speech.
The Brigadier: Colonel Mace, again. The actual Brigadier doesn't appear.
The Bus Came Back: Martha returns for a few episodes.
Cloning Blues: Martha's Evil Twin.
Continuity Nod to "Voyage of the Damned" and "The Runaway Bride."
- And "Dalek" (the "In-tru-da window" pun made by Henry van Statten).
Covered in Gunge in a distinctly non-humorous manner though.
Cursed with Awesome: The Sontarans' armor has only one weak point, in the back of their necks. For the Sontarans, retreat is not only dishonorable, but impractical.
Ditto Aliens: Though there are minor differences between Staal and Skorr, despite them being clones.

"How... do you tell each other apart?"
"We say the same of humans."

Every Car Is a Pinto & Overclocking Attack: Subverted, when the Atmos system in the Land Rover suffers from the Logic Bomb, the Doctor and Ross Jenkins run for cover... then the sat nav system fizzles and sparks a little bit.

The Doctor: [disappointed] Is that it?

Failed Attempt At Drama: Donna says she's going home. The Doctor gives her the big speech about everything he wanted to show her, how she's saved his life, how he was so very grateful for her company and... you're just popping home for a visit, aren't you?

Donna: You dumbo.

Fandom Nod to the UNIT dating controversy

"Back in the seventies.. or was it the eighties? It was all a bit more... homespun back then."

Foreshadowing: The ATMOS Sat Nav repeatedly saying "Turn Left".
- There's also the cloned copy of a person animated by the still-living person's memories.
Grammar Nazi:

Luke: If only that was possible.
The Doctor: If only that ‘’were’’ possible. Condition clause.

- Later...

Luke: You can’t say ATMOS system ‘cause it stands for Atmospheric Omission System. So you’re saying Atmospheric Omission System System.

Immune to Bullets: Sort of... The Sontaran's new Applied Phlebotinum prevents guns from being fired in their presence.
Informed Ability: The Sontarans being the 'universe's greatest soldiers' when their tactics appear to mostly consist of walking forward and shooting people. And unlike say, Daleks with powerful shields, these guys get cut down by bullets pretty easily when their copper-expanding gimmick fails.
Insufferable Genius: See Grammar Nazi for just one example of it.
Insult Misfire: Don't call Ross a grunt.
Proud Warrior Race Guy: All the Sontarans. Also Colonel Mace.
Red Hats
Take That: To either JNT or Colin Baker.

Doctor: Getting too close to the Eighties.
Donna: What am I gonna do, put a dent in it?
Doctor: Well, someone did.

Teen Genius: Luke Rattigan.
Where's the Kaboom?