S04E04: 'The Sontaran Stratagem'

A young genius called Luke Rattigan is working with the Sontarans by using their technology to devise a device called Atmos, which makes a car carbon-free and functions as a Sat Nav. However, 52 deaths across the world occur simultaneously when Atmos takes control of several cars.

The Doctor receives a call from UNIT employee and former companion Martha Jones, who is now engaged to Tom Milligan. The Time Lord and Donna head to Earth and meet Colonel Mace, who is spearheading a raid on an Atmos factory. Two UNIT soldiers find an underground chamber but are captured and cloned by the Sontaran General Staal, as is Martha.

Donna heads home to visit her family while The Doctor visits Rattigan with a soldier called Ross. The Doctor discovers a teleport device and momentarily finds himself on board the Sontaran ship before beaming back. General Staal follows him down to Earth, though The Doctor manages to hit a squash ball onto his vulnerable probic vent and briefly incapacitate him and escape. The Doctor and Ross then manage to fend off an attempt to kill them using the Atmos in their UNIT jeep.

The Doctor calls Martha and tells her to inform UNIT about the Sontarans and put them on Code Red, but the clone doesn't comply. Around the world, a poisonous gas emits from millions of Atmos devices after the Sontarans activate them. Wilf is trapped inside his car, choking, and The Doctor battles to free him.

> Click here for our review of 'The Sontaran Stratagem'