The Sontaran Stratagem

Martha Jones calls the Doctor to ask for assistance during an investigation by UNIT. Minutes after the TARDIS materialises in contemporary Britain, Martha authorises the raid of an ATMOS (Atmospheric Omission System) factory. The Doctor introduces his companion Donna Noble to Martha and UNIT; Donna instantly befriends Martha, but is concerned about UNIT's ethics and asks the Doctor why he is associated with them; the Doctor ambiguously replies he used to work for them in the late twentieth century.
ATMOS is marketing a satellite navigation system developed by child prodigy Luke Rattigan. The system also reduces carbon dioxide emissions to zero; UNIT requested the Doctor's help because the technology is not contemporary and potentially alien. UNIT are also concerned about fifty-two simultaneous deaths occurring spontaneously several days before the narrative. The Doctor travels to Rattigan's private school to investigate the system, and discovers that the episode's events are being influenced by the Sontarans.
The Sontarans depicted in the episode are part of a battlegroup led by General Staal, "the undefeated". Instead of an instant invasion, they are tactically approaching an invasion with a combination of human clones, mind control, and ATMOS; Martha is captured by two of the controlled humans and cloned to provide a tactical advantage against UNIT.
A subplot depicts Donna returning to her home to warn her mother Sylvia and grandfather Wilfred Mott about the Doctor, having been advised to do so by Martha. Concerned about the implications of telling the truth, Donna reneges from warning her mother. At the end of the episode, the Doctor investigates the ATMOS device attached to Donna's car and discovers a secondary function: the device can emit a poisonous gas. Wilfred attempts to take the car off the road, but is trapped when Staal activates all 400 million ATMOS devices installed in cars worldwide. The episode's cliffhanger depicts Donna shouting for help while the Doctor stares helplessly at a street full of cars emitting the gas, while on their ship orbiting the planet, the Sontarans prepare themselves for battle.

Planet of The Ood

The Doctor uses the TARDIS to land at a random point in time and space. On leaving the TARDIS, he and Donna find an injured Ood, a species the Doctor previously encountered in "The Impossible Planet" and "The Satan Pit". Just before he dies, the Ood's eyes turn red and it makes a lunge for the Doctor, startling him with his ferocity. The Doctor muses that they were being influenced by the Devil on their previous encounter, and concludes that on this occasion they must be being influenced by a different and closer being. The Doctor and Donna find an industrial complex controlled by Ood Operations, who have been selling the Ood as a servant race since 3914. They have even been making certain upgrades to their translation sphere. These include standard voice and a more seductive female voice. One even has been adapted with comical expressions such as "D'oh" from Homer Simpson of The Simpsons. The Doctor locates their position: the Ood-Sphere in 4126 close to the Sense-Sphere of the Sensorites.
The "Red Eye" phenomenon is affecting other Ood on the planet: several people have been killed in the weeks prior to the narrative. During the outbreak, the Ood state that "the circle must be broken". Ood Operations noted an increase in the phenomenon, and considered it to be similar to foot-and-mouth disease; CEO Klineman Halpen tells the Doctor the method of killing is identical.
Throughout the episode, Donna becomes sympathetic to the Ood and is horrified by their status as slaves. The Doctor also takes an interest in the Ood, noting that no species could naturally evolve to serve. He also feels he had overlooked them on their previous encounter. He and Donna travel through the complex and find a batch of uncultivated Ood. Instead of a translation sphere, they hold a "hind brain" that gives them individuality, and once removed, they become subservient; the Doctor castigates Halpen for lobotomising them.
The Doctor and Donna are captured by Ood Operations' security force. Shortly after, the Ood begin a mass revolution, and the complex is evacuated. The Doctor follows Halpen to a locked warehouse. The warehouse contains a large brain, which completes the Ood's collective consciousness. The brain's control of the Ood is limited by a circle of pylons emitting a forcefield. Halpen plans to kill the brain, and by extension, all of the Ood, but is stopped by a joint effort between the Doctor, Donna, Dr Ryder, and Halpen's personal Ood, Ood Sigma; Ryder, an activist for "Friends of the Ood", had slowly infiltrated the company over the course of ten years before he was able to gain access to the controls for the pylons and change them to their minimum setting, while Ood Sigma used Halpen's hair-loss medication to slowly convert Halpen into an Ood.
The Doctor shuts down the circle, freeing the Ood and allowing them to all rejoin in a telepathic collective. Before leaving, Ood Sigma promises to include the "Doctor-Donna" in the Ood's song; stating that "the Wind, the Ice and the Snow" shall remember and honour their names forever, but comments that the Doctor's song may soon end.

The Fires of Pompeii

The Doctor and Donna Noble arrive in what the Doctor believes to be Rome in the first century AD. After an earthquake and witnessing a nearby mountain begin to smoulder, he realises he has in fact materialised in Pompeii on 23 August, AD 79, one day before the eruption of Mount Vesuvius. When he and Donna returns to the TARDIS' location, he is told it was sold to a Lucius Caecilius Iucundus, a marble sculptor.
The episode's antagonists are the Pyrovile, giant rock-like creatures resembling golems whose home planet was "lost". They operate secretly; the Sybilline Sisterhood act as their proxies. They use the Sisterhood, which incorporates a high priestess and her acolytes, Spurrina, and Thalina (Lorraine Burroughs), to make prophecies while slowly converting them to stone. The Sisterhood is inducting Caecilius' daughter Evelina and is allied to the local augur Lucius. The Doctor is disturbed by their knowledge of his and Donna's personal lives, and by Lucius' latest commission, a marble circuit board.
The Doctor breaks into Lucius' home and discovers that he is creating an energy converter. He is accosted by Lucius, who sends a Pyrovile to kill the Doctor. The confusion allows the Sisterhood to kidnap Donna briefly; the Doctor follows them and frees Donna. They escape into the Sisterhood's hypocaust system and travel into the centre of Mount Vesuvius.
Mount Vesuvius is being used by the Pyrovile to convert the human race to Pyroviles, in an effort to conquer Earth. The Doctor realises the volcano will not erupt if the energy converter is running, and with Donna's encouragement, subsequently switches it off, triggering the eruption of Vesuvius, considering Pompeii's destruction and the death of its population the lesser of two evils. The Doctor attempts to leave, but Donna convinces him to save Caecilius and his family, whom he then takes on board the TARDIS. The family, The Doctor, and Donna then watch Pompeii's destruction from a vantage point. The Doctor assures the family that Pompeii is never forgotten before leaving with Donna. As he watches the destruction, Caecilius comments that it is the wrath of Vulcan and coins the word "volcano" to describe it.
The last scene takes place six months later in Rome. Caecilius' family are shown to be successful: Caecilius is running a profiting business, Evelina has a social life in comparison to her seclusion in Pompeii, and his son Quintus (Francois Pandolfo) has given up the wasteful life he led in Pompeii and is training to become a doctor. Before Quintus leaves, he pays tribute to the family's household gods, whose statues are in the form of the Doctor, Donna and the TARDIS.

Partners In Crime

The episode primarily focuses on Donna Noble, a previous companion who appeared in "The Runaway Bride". After her encounter with the Doctor, she became disenchanted with normal life and regretted declining his invitation to travel in the TARDIS. She started investigating conspiracy theories in the hope she would find him. She confides her regrets to her grandfather Wilfred Mott, an amateur astronomer who met the Doctor before in "Voyage of the Damned".
The episode concerns Adipose Industries, which is marketing a diet pill to London's population with the slogan "the fat just walks away". Believing the treatment to be otherworldly, the Doctor and Donna investigate the company separately, and find that the slogan is literal—the pills use latent body fat to parthenogenetically create the Adipose, small white aliens which spawn every night, removing a little of the host's body fat each time. In an emergency, multiple Adipose can spawn by using all of the body's organic tissue, killing the host. When the Doctor and Donna meet, they are confronted by Miss Foster, an alien who is exploiting Britain's overweight population to create the Adipose for the Adiposian First Family. Miss Foster mentions that the Adipose species "lost" their breeding planet and hired Foster to find a replacement; she chose Earth, knowing that it was illegal.
Foster accelerates her plans, feeling threatened by the Doctor's invocation of galactic law and fearing he may inform the "Shadow Proclamation", an interplanetary police force. Throughout London, the Adipose begin to spawn, soon numbering several thousand. The Doctor and Donna prevent total emergency parthenogenesis occurring, while the remainder make their way to Adipose Industries. The Adiposian First Family use their spaceship to collect the Adipose, but kill Foster to hide any evidence they used Earth illegally. The Doctor refrains from killing the Adipose because they are children; Donna notes that his previous companion Martha Jones made him more human, citing his infanticide of the Racnoss in their previous encounter.
At the end of the episode, Donna accepts an offer to travel in the TARDIS. She makes a detour to leave her car keys in a safe location for her mother Sylvia, and asks a blonde woman to help Sylvia find the keys. The woman turns towards the camera, revealing her to be Rose Tyler. She fades from view as she walks away from the area. In the final scene, Donna asks the Doctor to fly by her grandfather, Wilfred, who sees her and celebrates on his allotment.