Cars that kill, kids too clever for their own good, and an army of space spuds with superiority complexes.
Short Synopsis
The Doctor, Donna, and a returning Martha Jones uncover a plot by the Sontarans to use a revolutionary eco-car system to suffocate Earth. Meanwhile, UNIT tries to be helpful but mostly provide target practice.
Extended Storyline
Donna, still adjusting to the whirlwind of TARDIS life, takes a trip back home to see her family. She’s barely had time to explain “Yes, Mum, I travel with an alien, no, I’m not pregnant” when the Doctor gets a call from UNIT. (Because if there’s anything the Doctor can’t resist, it’s men in uniforms saying, “We’ve got a thing we don’t understand, please sort it out.”)
Enter ATMOS — a new car navigation and emission system, invented by boy-genius Luke Rattigan, who looks like the sort of kid who’d get bullied in school and then grow up to hold TED Talks about “disrupting industries.” His ATMOS units have been installed in basically every car on Earth, making him both rich and suspicious. Spoiler: never trust a teenage CEO.
Donna tags along to investigate, where they meet Martha Jones — now working with UNIT, rocking a lab coat, and radiating “I’ve moved on, Doctor” energy. Donna is thrilled to meet another companion, while the Doctor looks like he’s about to be caught cheating.
Meanwhile, sinister things are afoot. People with ATMOS in their cars are being suffocated by toxic gas emissions. UNIT soldiers investigating disappear mysteriously. And Luke Rattigan is holding science camp for hand-picked geniuses, all of whom seem disturbingly okay with their field trip leaders being militaristic potatoes.
Yes, the Sontarans are back, led by General Staal, a warrior so proud of his martial heritage that he practically salutes himself in the mirror every morning. They’ve partnered with Rattigan, who thinks he’s helping terraform Earth for his chosen nerd army to inherit. In reality, the Sontarans are just using him as a pawn in their latest attempt to conquer everything not nailed down.
The Doctor investigates one of the ATMOS-fitted cars, only to be ambushed by UNIT soldiers now under Sontaran control. They’re turning humans into mindless drones, because apparently evil plots need minions who aren’t just squishy.
Meanwhile, Donna visits home again in one of the best companion-family scenes ever. She breaks down in her grandfather Wilf’s arms, terrified of all she’s seen, and for the first time we get a raw glimpse of Donna’s humanity clashing with the enormity of traveling with the Doctor. (Wilf, naturally, is just delighted that his granddaughter is off adventuring with aliens. Best grandad ever.)
The episode builds to the big reveal: ATMOS wasn’t just a clever green car gimmick — it’s part of a global plot. At the Sontarans’ command, millions of cars activate simultaneously, belching toxic gas into the atmosphere. The planet begins to choke, UNIT panics, and the Doctor finally realizes this isn’t just a military flex — it’s an attempted full-scale planetary genocide.
Cue cliffhanger: the Earth literally suffocating under a blanket of poison, the Doctor glaring at a smug Staal, and Donna stuck in the middle of it all wondering if she should’ve stayed home with her mum.