Martha Jones (Freema Agyeman), former companion of the Doctor (David Tennant) is working for UNIT (Unified Intelligence Taskforce). She calls the Doctor for assistance during an investigation. As soon as the Doctor’s TARDIS materialises in the UK, Martha announces the raid of an ATMOS factory. The Doctor introduces Donna (Catherine Tate) to Martha and UNIT. Donna befriends Martha but is not sure about the ethics of UNIT.

ATMOS is selling a satellite navigation system developed by a young genius named Luke Rattigan (Ryan Sampson), which reduces carbon-dioxide emissions to zero, but UNIT thinks the technology might be alien. Also, there have been 52 mysterious deaths. While everyone is studying the technology at the plant, Donna checks out the personnel office and discovers there have been no sick-days taken by anyone in the plant.

The Doctor travels to Rattigan’s private school to investigate the system and discovers that the militarist alien Sontarans are involved. They are led by General Stahl the Undefeated (Christopher Ryan) and Luke is working for him. Instead of invading Earth, they are taking control with human clones, mind-control, and ATMOS. Martha is captured and cloned, and her clone is installed as a mole within UNIT.

Donna returns home to explain to her mother Sylvia (Jacqueline King) and her grandfather Wilfred (Bernard Cribbins) that she has been travelling in time and space with the Doctor but decides not to tell her mother after all. The Doctor investigates the ATMOS device attached to Donna’s car and discovers it can release poison gas. General Stahl activates 400 million ATMOS devices. To be continued.

Martha is more mature now, as a UNIT medical officer, and no longer smitten with the Doctor. It was decided not to make a big deal of her relationship to Donna. It would have been too reminiscent of Rose Tyler’s (Billie Piper) relationship with Sarah Jane Smith (Elisabeth Sladen) in a previous episode. Catherine Tate thought the Sontarans were artificial and did not realize there were actors inside until she was freaked out by one of them removing his helmet. Freema Agyeman enjoyed playing an evil companion.

This was the first Sontaran story in 23 years. The Doctor says he had worked with UNIT in the 1970s—or was it the 1980s? There has always been confusion about whether UNIT stories took place contemporaneously with the broadcast or took place in the near future. Dan Starkey (Commander Skorr) would re-appear as a Sontaran in many future stories. This episode was described as “deliciously old-fashioned Doctor Who.” It was criticized as slow, but it was the first of a two-parter and there was a lot of set-up for Part Two.