A Call for Help

UNIT is investigating ATMOS, a new carbon-cleaning car filter/GPS navigation system that has been fitted to millions of cars across the whole world, and their newest recruit - Martha Jones, now a fully-credentialed physician - calls The Doctor back to Earth for assistance as they need his help in determining if the ATMOS system, in the factory they have just raided, is alien in origin. Though it is much more advanced than human technology should be, The Doctor confirms that it is entirely human-made. However, his current companion, Donna Noble, soon realises that while the system may be man-made, the factory workers don't seem to be human, as not a single one has ever taken a sick day.

While they prepare to examine the workers, two UNIT soldiers stumble upon a cloning facility in the basement. There they are confronted by a Sontaran, who disables them and has them hypnotically converted into loyal servants. They in turn kidnap Martha, who is cloned by the Sontaran and kept prisoner so the clone can access her memories.

Meanwhile The Doctor is checking out Luke Rattigan, a child genius who is purported to have invented ATMOS. At Rattigan's school The Doctor discovers that Luke Rattigan is working with the Sontarans when he spots a teleportation device which, when he uses, teleports him to the Sontaran's battleship. Realising the danger he is in he quickly teleports back and then disables the device – but not before a Sontaran follows him through. It is then that The Doctor discovers the Sontarans plan to use ATMOS to turn four hundred million cars into deadly weapons.

Martha Jones

The Sontarans on realising they are dealing with The Doctor attempt to make the UNIT ATMOS-equipped jeep drive into the river, but he uses a logical paradox to overload the system. The Sontarans then accelerate their plans and so they activate converters in each ATMOS system which starts to pump out poisonous gas.

Realising what is happening The Doctor tries to warn UNIT, but since it is the Martha clone he calls, they remain uninformed. Meanwhile, Donna's grandfather is trapped in his car, and despite their best efforts The Doctor and Donna cannot free him. The Doctor finds himself powerless as hundreds of cars release the poison into the atmosphere.

The Doctor and Donna

The Doctor though is more concerned about what the Sontarans are up to especially as it is not like them to fight their battles in this way. The Doctor is about to find out what the Sontarans are planning to do with the Earth once the human population has been dealt with.

How is The Doctor going to stop the death of billions of people and to stop the Sontarans from turning the planet into a giant breeding world for more clones? First though he has to stop UNIT from using nuclear weapons against the Sontarans, rescue Donna - who is now in the TARDIS which has been teleported aboard the Sontaran’s space ship – and to find out what has happened to the real Martha?

But before The Doctor can do anything a battalion of Sontarans teleport to the planet so as to secure the ATMOS 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 back in control of the Sontarans.

Martha is Back

As UNIT begins a counterattack, loading their weapons with non-copper bullets and using the aircraft carrier Valiant to clear the gas, The Doctor is able to use the distraction to make his way to the cloning room where he severs the cloned Martha’s connection to the real Martha - so killing the clone.

The Doctor then builds his own atmospheric converter, igniting the atmosphere to clear out the poison gas just before it gets to a critical density. However, he is in a race against time as he knows that the Sontarans won't accept defeat so easily and so he teleports to their ship with the converter, planning to give them the choice between retreat or death. The Sontarans choose the latter, forcing The Doctor to sacrifice himself. Help though is at hand when Rattigan decides to make amends for the damage he has caused, by assisting the Sontarans, when he teleports himself to the Sontaran ship and brings The Doctor back to Earth, sacrificing himself instead so as to destroy the Sontaran’s space ship.

With the Sontarans finally defeated, and the world saved, The Doctor and Donna return to the TARDIS along with Martha. But Martha has no intention of continuing her travels in the TARDIS. But as The Doctor and Donna say their goodbyes the TARDIS springs to life and begins piloting itself before Martha can leave.