DOCTOR WHO IN DETAIL

A journalist, investigating Atmos, is discovered and chucked out of the Rattigan Institute she had been investigating. Whilst making her way to UNIT HQ the journalist is led to a river by her sat-nav system. Her car is equipped with Atmos, and it takes over her car and drives into the river.

In the TARDIS the Doctor is teaching Donna to fly the TARDIS when a mobile phone rings - the phone that Martha left behind. Martha is waiting where the TARDIS materialises.

Martha is a fully fledged Doctor now, working for UNIT. UNIT has been called in to raid an Atmos factory. UNIT are anxious to meet the Doctor, but he is not as anxious to meet them. The Doctor is introduced to Colonel Mace.

UNIT are investigating 52 deaths worldwide that happened at the same time in different time zones. The link is that all the deaths were in cars equipped with Atmos. The Doctor doesn't know what Atmos is, but Donna does. Atmos is a system that reduces a car's emissions to zero, and has bonus features such as sat-nav. An alien involvement is suspected.

In a lower level of the factory two UNIT soldiers come across a guarded area. Under orders the guards let the UNIT soldiers inside a darkened room. When the lights are turned on they find strange equipment including a covered vat. Banging comes from inside the vat. One of the soldiers tries to open the vat thinking there is some trapped inside. When it is opened a humanoid figure emerges from the green liquid inside.

Behind the soldiers a small squat creature in armour apepars. When provoked the creature fires at one of the soldier's knees. The creature disables the soldiers and orders the guard to take them away. Before they go the creature tells them who it is - General Staal of the 10th Sontaran fleet.

Whilst UNIT has been investigating the factory Donna, a former temp, has been investigating personnel, and finds that the personnel folder of sick days that contains no files. None of the staff have ever taken a sick day. Based on this Martha is told to start testing the staff.

Colonel Mace tells the Doctor about the Luke Rattigan, inventor of Atmos, and the Rattigan Institute - a school for geniuses - that he founded. The Doctor is keen to visit, and takes a jeep with a UNIT soldier - Ross.

Martha is examining the staff. The first factory worker examined says he works 20 hours a day, and has a racing heartbeat, making her suspicious. When she tries to talk see the colonel she is led in a different direction by the two UNIT soldiers from the lower level of the factory. They lead her back into the guarded room.

At the Rattigan Institute the Doctor starts looking round some of the equipment they have. The Doctor starts to annoy Luke Rattigan by correcting his diction and pointing out mistakes. The Doctor notices a teleport system, and teleports himself to a Sontaran ship. The Sontaran follows the Doctor back through the teleport. The Doctor uses a racket to bounce a squash ball from the teleport and this his Staal on his probic vent. This is a weakness of the Sontarans and causes Staal to collapse, and the Doctor and Ross manage to escape.

In the factory Martha meets Commander Skarr as she is tied down. Her image, and brainwaves, are being copied to the creature in the vat. As Martha watches a clone emerges from the tank as Martha looses consciousness.

On the Sontaran ship Luke mentions the Doctor, an enemy of the Sontarans. The jeep the Doctor in has Atmos installed. The sat-nav says to turn left, and Ross is unable to do anything to stop it. The jeep is heading towards a river as the Doctor asks the Atmos unit if it is programmed to not as he says. When the Atmos unit confirms this the Doctor orders it to drive into the river. As it approaches the jeep stops. On the ship the Sontarans think the Doctor is dead.

Donna has gone home to visit her Mum, and the Doctor calls in on her. The Doctor starts to disconnect the Atmos unit from Martha's car. The Doctor activates the Atmos, and gas starts to emit from the unit. The Sontarans see a unit has been activated, and realise the Doctor must still be alive. The realisation dawns that 400 million cars on earth have gas emitters in them, and the Sontarans send the activation signal.

The Martha clone uses her clearance to access the NATO defence system and transfers security protocols to a memory card. Using a PDA she sends the codes to the Sontarans. As cars worldwide start to emit toxic gas a state of emergency is announced.

Ross manages to find a old London black taxi cab without an Atmos unit, and the Doctor, Donna and Ross return to the Atmos factory. Donna returns to the TARDIS where she is out of the gas. When the Doctor enters the factory office the Martha clone alerts the Sontarans. They teleport the TARDIS, with Donna inside, to their ship. The Doctor and the Martha clone reach the spot where the TARDIS was. The Doctor realises that Martha is behaving differently and has not mentioned her fiance despite all the gas.

UNIT has analysed the gas and found out the gas is not deadly until it reaches 80% density. The Doctor opens a video link to the Sontaran ship, and begins to provoke them. General Staal will not reveal his true plan, and shows they have captured the TARDIS. The Doctor starts to drop hints for Donna, who is inside.

UNIT, despite the Doctor's instructions, decide to launch a nuclear strike against the Sontarans. When the countdown reaches zero the Martha clone cancels the launch. Staal orders Sontarans to protect the Martha clone. The Sontarans take back the factory as the UNIT soldiers retreat.

Luke Rattigan had been promised passage to another planet for him and his students by the Sontarans. Returning to the Sontarans' ship he learns that they would have been killed, and that he had been used by them. He activates the teleport and returns to Earth as the Sontarans open fire.

The Doctor calls Donna from a mobile phone. The Doctor asks Donna to reinitialise the Sontarans' teleport system. There is a Sontaran outside with his back to the TARDIS. Donna uses the TARDIS mallet to strike the probic vent, knocking out the Sontaran. The Doctor talks Donna through the Sontaran ship.

Outside the factory UNIT orders in the carrier Valiant. From the air the jets clear away the gas, and the ship fires on the factory. UNIT start to retake the factory, using non-copper bullets to get round the Cordolaine signal of the Sontarans.

The Doctor and Martha clone go to the room containing the real Martha. As the Doctor examines Martha the clone draws a gun. The Doctor reveals he knew about her from the start, and that he let her stop the nuclear launch as he wanted that as well. The Doctor frees Martha, and the clone collapses when disconnected from Martha. As the Doctor starts to repair the teleport, Martha talks to her clones, and convinces the clone to help, The Martha clone reveals what is in the gas. The Sontarans are trying to convert Earth's atmosphere to be suitable for breeding their clones.

On board the Sontarans' ship Donna re-activates the teleport, and the Doctor teleports Donna and the TARDIS back to Earth.

Inside the Rattigan Institute the Doctor assembles an atmosphere converter. The Doctor activates the converter, and ignites all the gas in the atmosphere. The gas is burnt away in an instant. The Sontarans revert to Stratagem One and prepare to destroy Earth. The Doctor realises there is only one way to stop the Sontarans: ask them to leave or use the converter to destroy their ship.

He teleports back and gives the Sontarans an ultimatum. The Sontarans refuse, and are prepared to die. At the last moment Luke teleports to the Sontarans' ship, teleports the Doctor back, and fires the converter, destroying the Sontarans before they can destroy Earth.

Inside the TARDIS the Doctor and Donna are saying goodbye to Martha when the TARDIS takes off...