Following from the previous episode, Sylvia Noble (Jacqueline King) manages to free Wilfred Mott (Bernard Cribbins) from the car by smashing the window with an axe. The Doctor (David Tennant) sends Donna Noble (Catherine Tate) back to the TARDIS while he sets off to figure out what the Sontarans are up to. After studying the gas, UNIT determines that it will need to reach 80% density to become lethal. Elsewhere, Martha Jones's clone (Freema Agyeman) informs the Sontarans of UNIT's defence systems and helps them to teleport the TARDIS to their orbiting ship. Realising that he is trapped, the Doctor attempts to goad General Staal (Christopher Ryan) into revealing their plan: Staal is smart enough not to fall prey to this ploy, but the Doctor does trick him into moving the TARDIS out of the main war room, placing Donna in a position to help.
Against the Doctor's advice, UNIT decides to use nuclear weapons against the Sontarans; however, Martha's clone has covertly copied the launch codes, and stops every attempt they make to fire the weapons. This in itself shows a hidden agenda, since the Doctor knows a nuclear strike would not have harmed the Sontarans in the first place. This, combined with the unidentifiable elements in the gas, suggest that the Sontarans have an interest in keeping anything from disrupting the atmospheric conversion. At the same time, the Sontarans, under Commander Skorr "the Bloodbringer" (Dan Starkey), mobilize a contingent of troops to protect the factory. With the Sontarans' ability to jam most conventional firearms by expanding the copper-lined bullets, the UNIT troops are quickly slaughtered and the factory is secured.
Luke Rattigan (Ryan Sampson) leaves the Sontaran mothership to gather his students, explaining that he plans to have the Sontarans take them to another planet and begin the human race anew. The students merely laugh him off, even when he brandishes a gun. When he returns to report his failure, the Sontarans likewise ridicule his efforts, admitting that they had planned to kill his students and never intended to take him anywhere. Rattigan teleports back to his mansion before they can kill him, and the Sontarans lock the teleport pods behind him. Back in his own quarters, he lies sobbing on the ground.
Meanwhile, the Doctor instructs Donna on how to reopen the teleport pods. As she makes her way through the ship, UNIT begins a counterattack, loading their weapons with non-copper bullets and using the aircraft carrier Valiant to clear the gas. The counterattack is a success, and the UNIT troops are able to put the Sontarans on the defensive. The distraction allows the Doctor to make his way to the cloning room where Martha is being held. Having figured out long before that the clone wasn't the genuine article, he severs its connection to Martha, leaving it to die. Martha convinces the clone to betray the Sontarans in its last moments, and the clone reveals that the poison gas is actually "food" for Sontaran clones: they are converting the planet into a giant breeding world. With Donna's help, the Doctor is able to reactivate the teleport pods, allowing him to rescue Donna, steal back the TARDIS, and teleport into Rattigan's mansion.
With the terraforming equipment Rattigan's students built, the Doctor builds his own atmospheric converter, igniting the atmosphere to clear out the poison gas as shown in the picture. However, he knows the Sontarans won't accept defeat so easily, and teleports to their ship with the converter, planning to give them the choice between retreat or death. Staal chooses the latter, content with the knowledge that the Doctor will die with them. At the last moment, Rattigan teleports himself to the Sontaran ship and sends the Doctor back to Earth, sacrificing himself to destroy the Sontarans.
With the day saved, Martha says goodbye to Donna and the Doctor in the TARDIS and prepares to head home. However, before she can leave, the TARDIS suddenly springs to life, locking the doors and piloting itself to an unknown destination as the jar containing the Doctor's severed hand bubbles.
Doctor David Tennant (Tenth Doctor)
Companions Catherine Tate (Donna Noble)
Freema Agyeman (Martha Jones)
Billie Piper - Rose Tyler
Christopher Ryan – General Staal
Rupert Holliday-Evans – Colonel Mace
Bridget Hodgson - Captain Price
Dan Starkey – Commander Skorr
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Ryan Sampson – Luke Rattigan
Christian Cooke - Ross Jenkins
Clive Standen - Private Harris
Wesley Theobald - Private Gray
Meryl Fernandes - Female Student
Leeshon Alexander - Male Student
Kirsty Wark - Herself
Lachele Carl - US Newsreader
Jack Steed - Lieutenant Skree
Writer Helen Raynor
Director Douglas Mackinnon
Script editor Nikki Smith
Producer Susie Liggat
Executive producer(s) Russell T. Davies
Production code 4.5
Series Series 4
Length 2nd of 2-part story, 45 minutes
Originally broadcast 3 May 2008