(He uses the sonic screwdriver to connect the UNIT systems to the Sontaran ship). An atmospheric converter. Colonel Mace returns to the UNIT base. DOCTOR (shouting from the teleport pod): The gas! DOCTOR: But the Sontarans have got lasers! DOCTOR: I've got the TARDIS, I'm gonna get on board their ship. DONNA: Why not? GENERAL STAAL: Execute him! He teleports back to Earth. I'm on it. In the clone lab, the Doctor is working on the teleport while the real Martha sits beside the clone. It can't be stopped, it's everywhere. MARTHA CLONE: But... where's the TARDIS? Over. Retreat. SONTARAN: A human female on board ship. SYLVIA: Donna! She picks up the mallet that the Doctor uses so happily when piloting the TARDIS. Showing it to me. Standing up, he pushes a key and teleports to the Sontaran ship, at the same time zapping the Doctor back to the Academy. Tell them to stay inside. Tell us about the gas. The Doctor and the Martha clone enter the UNIT base. The Doctor appears on the Sontaran ship. The Doctor gets inside the teleport pod with the atmospheric converter. The Doctor runs down the alleyway to find the TARDIS gone. The TARDIS is teleported to the Sontaran ship, with Donna inside. The taxi pulls up to the ATMOS factory, the Doctor and Donna exit.
Plan: A: Sylvia; Q: Who frees Wilfred? A: The Doctor; Q: Who constructs an atmospheric converter at Rattigan's academy? A: Donna; Q: Who does the Doctor tell to re-engage the teleport pods? A: the TARDIS; Q: What is the name of the Doctor's time machine? A: a counterattack; Q: What does UNIT manage? A: the real Martha; Q: Who does the Doctor awaken in the factory? A: Martha's clone; Q: Who tells the Doctor that the gas is being used to convert Earth into a breeding world for the Sontarans? A: Rattigan; Q: Who sacrifices himself to destroy the Sontarans? A: the humans; Q: Who does the Doctor's atmospheric converter allow to breathe? A: Staal; Q: Who does the Doctor offer the chance to retreat? A: places; Q: Rattigan switches __________ with the Doctor. Summary: Sylvia frees Wilfred. The Doctor and Donna return to the ATMOS factory, where the Doctor warns UNIT not to engage the Sontarans. The Doctor tells Donna to stay in the TARDIS, but the Sontarans teleport the TARDIS aboard their ship. The Sontarans defeat UNIT at the factory and take it over. UNIT manages a counterattack. Finding the TARDIS missing, the Doctor tells Donna to re-engage the teleport pods. The Doctor enters the factory and awakens the real Martha. The Doctor learns from Martha's clone that the gas is being used to convert Earth into a breeding world for the Sontarans. The Doctor tells Donna how to use the pods and teleport the TARDIS to Earth. The Doctor constructs an atmospheric converter at Rattigan's academy, which harmlessly ignites the gas and allows the humans to breathe. He calibrates the converter so it can ignite the Sontarans on board their ship, and teleports on board. The Doctor offers Staal the chance to retreat, but Staal encourages him to destroy them. Rattigan switches places with the Doctor, sacrificing himself to destroy the Sontarans. Martha is trapped on the TARDIS when it takes flight on its own.