Doctor Who Series 4 - The Sontaran Stratagem The Poison Sky Trailer

UNIT is investigating ATMOS, a new device which somehow cleanses the exhaust from automobiles. The Taskforce believes that ATMOS is alien technology, and so their newest recruit -- Martha Jones, now a fully-credentialled physician -- calls the Doctor back to Earth for assistance. ATMOS is purported to be the invention of child genius Luke Rattigan, but the Doctor soon discovers that Rattigan is working with the Sontarans, who have used ATMOS to turn four hundred million cars into deadly weapons.