Series 4 (2008)
Episode title: The Sontaran StratagemFeaturing: Tenth Doctor
Companions: Donna Noble, Martha Jones
First shown: 26th April 2008
Related articles: Sontarans
Guide: This episode starts with the Doctor and Donna in the TARDIS, when the Doctor's hears a ringing phone. He answers it and he remembers it's the one Martha Jones gave him at the end of series 3 (episode 13, Last of the Time-Lords). Martha is ringing him, calling him back to Earth.
The Doctor and Donna land on Earth, and the moment they step out of the TARDIS Martha is there, and she starts the invasion of an ATMOS (Atmospheric Ommisions System) factory by UNIT (formally the United Nations Intelligence Taskforce, now the Unified Intelligence Taskforce). It is revealed that Martha is a high ranking medical person within UNIT, who got the position after someone in UNIT received a call from a higher up body (this was also mentioned in the Torchwood episode, Reset, when Martha joins Captain Jack Harkness. Jack mentions that it was probably the Doctor who called UNIT, but there is no evidence to prove this).
After the invasion of the factory, the Doctor is brought up to date by Martha and her boss, that the ATMOS may be alien origin. The Doctor goes to its creators mansion, Luke Rattigan - child genius.
But Luke does not reveal anything about the ATMOS, but when the Doctor is driving back to the factory, the car starts driving itself, into the canal. The Doctor stops it, and the car gives off some sparks in way of explosion.
Meanwhile, Martha has been taken hostage by two cloned UNIT soldiers, and she is then cloned herself, to get information from the UNIT for the enemy. The Doctor returns to the factory, and discovers that the enemy in question is the Sontarans. Led by General Staal 'the undefeated' to invade Earth.
It then turns out that the ATMOS holds poisonous gas, which the Sontarans release.
The Doctor also meets Donna's family, but her grandfather, Wilf, gets trapped in his car when the ATMOS activates. The Doctor tries to get him out, and this is the cliff-hanger for episode 5 to finish.
This is the first of a two part story. Read the second part here.