Parts: 2 (45-minute)
Writer: Helen Raynor
Airdate: 26 April - 03 May 2008
Production Code: n/a

Doctor: tenth
Companion: Donna, Martha

Creature Feature: Sontarans
Special Guests: UNIT

Story: Young genius Luke Rattigan, head of the Rattigan Academy for exceptionally gifted students, has developed a brilliant new piece of technology called Atmos. Part GPS-navigation system and part environmental control, Atmos has the added benefit of reducing automobile carbon emissions to zero; as a result, Atmos has been wired into cars around the world, and even comes as a standard fit on new vehicles. To the world at large, this sounds like a miracle that will hugely lessen air pollution. But when 52 people die simultaneously, around the world, all in cars fitted with Atmos, UNIT gets suspicious.

New staff doctor Martha Jones uses her old cell phone to contact the Doctor, and ask for his help with the Atmos situation. Although they've never been able to find anything sinister about Atmos, UNIT is suspicious that it may be linked to alien involvement, and they need an expert to help them investigate it. When the Doctor travels to Rattigan Academy to talk to the inventor of Atmos, he discovers something unusual: a teleport pad in the middle of one of the rooms. And on the other end of the teleport is a Sontaran warship... but why are the Sontarans involved with Atmos, and what interest do they have in cleaning up the Earth's atmosphere?