400 million cars on Earth are suffocating humans with poisonous gas emitted from their ATMOS devices. The Doctor (David Tennant) tells Donna’s family to get inside and seal the windows and doors as well as they can. They are Donna’s mother Sylvia (Jacqueline King) and her grandfather Wilfred Mott (Bernard Cribbins). The Doctor and Donna (Catherine Tate) return to the ATMOS factory, where he warns UNIT not to engage the Sontarans. The Doctor tells Donna to stay in the TARDIS for her safety, but the Sontarans teleport the TARDIS to their ship.
The Sontarans prevent UNIT from launching a nuclear missile at them by using their clone of UNIT agent Martha (Freema Argyeman). The Sontarans take over the factory, overwhelming the UNIT troops. UNIT calls in the aircraft sky-carrier Valiant. The Doctor sneaks into the factory and discovers the real Martha. He awakens her, which kills the clone. Before dying, the clone reveals that the gas is clone-feed and is breeding billions of Sontaran soldiers. The Doctor tells Martha to stop UNIT from launching missiles and runs off.
Assisted by Donna, he uses the teleport to return the TARDIS to Earth. They teleport with Martha to Rattigan’s school and find him distraught over the damage he has caused. The Doctor builds an atmospheric converter which ignites the poison gas and allows the humans to breathe. Somehow this does not set the atmosphere on fire and the Doctor seems relieved. He offers the Sontarans an opportunity to withdraw, but General Stahl (Christopher Ryan) calls his bluff. Rattigan reactivates the teleport and switches places with the Doctor. He is destroyed with the Sontarans. Martha is just saying goodbye to the Doctor and Donna when the TARDIS doors shut, and it heads off to an unknown destination.
Donna Noble gets a key to the TARDIS. When the Doctor puts on a gasmask, he quips, “Are you my Mummy?” Donna knocks out a Sontaran with a wooden mallet that hangs from the TARDIS console. There is another mysterious appearance of Rose. The clone of Martha is obvious to the Doctor because of reduced iris contraction and a bald spot. Also, she smells funny. Brigadier Lethbridge-Stewart is stranded in Peru—a recurring joke from The Sarah Jane Adventures. The Sontaran/Rutan War, first mentioned, I believe, in Horror of Fang Rock with Tom Baker, has been going on for 50,000 years. ATMOS is Atmospheric Omission System.
Donna says she can’t open a door because it’s keyed to a three-fingered hand, and the Doctor points out that she has at least three fingers, so she makes a Vulcan salute. But then, the Doctor apparently doesn’t seem to realize, when Donna’s grandfather is trapped in a car, that windows are easily broken, until her Mum smashes it with an axe. Seems to me a sonic screwdriver would do nicely in such a situation, if you don’t have a soprano handy. The Sontarans are supposed to be great and noble warriors, and they are doing fine fighting from their ships with poison gas, but fighting hand-to-hand on Earth, UNIT mops up the floor with them. At first, the Sontarans use a device that prevents the UNIT guns from firing their copper-jacket bullets, but then UNIT switches to steel-jacket bullets and wipe them out. It’s nice to see UNIT finally battling somebody not immune to bullets, as Lethbridge-Stewart complained, but the Sontarans don’t seem like much of an army, for all their bluster.