Martha Jones, now working for UNIT, calls the Doctor (via her cell phone last seen in the season three finale, "The Last of the Time Lords") to help investigate an ATMOS factory. ATMOS (Atmospheric Omission System) is a device that has been installed and integrated on half of the world's vehicles. Besides acting as a GPS, ATMOS also reduces carbon dioxide emissions. What brings it to UNIT's attention is that 50-plus deaths have occurred in cars with ATMOS installed. Martha believes they were poisoned; however the toxin leaves the system immediately after death. As a result, UNIT believes the technology is alien in origin. The Doctor goes off to investigate the boy genius creator of the device, Luke Rattigan, who runs a private school. After a brief talk with Martha, Donna decides to visit her family instead of going with the Doctor to the academy. Donna reveals her secret to her grandfather but not initially to her mother. In the meantime, Martha is lured away from her duties by two...
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