The Doctor and Donna are called back to Earth by Martha, now is big wig at UNIT.  They are concerned with Atmos, a carbon dioxide remover now installed in over 400 million cars worldwide.  Over fifty people died at the exact same second and they need an expert to determine what is the cause - thus the Doctor.  During the investigation at the Atmos factory, Martha is cloned and is used to infiltrate UNIT and NATO but the Sontarans.  Meanwhile Donna goes home to see her mother and grandfather while the Doctor pays a visit to Rattigan where the genius behind Atmos lives and works.  He learns by looking around their lab, that they have a transmat device and uses it to find out about the Sontarans.  The Doctor goes to get Donna and his tinkering with an Atmos device in her mother's car triggers the Sontarans to start their plan.  All cars fitted with the devices begin to emit toxic fumes, and Donna's grandfather is stuck inside the car.