Inside Luke's laboratory the doctor finds,among other ingenious scientific discoveries, a teleporation portal that takes him on board a Sontaran vessel to meet with General Stall where their plan is revealed to destroy the people of Earth-having been excluded from the last great time war that the doctor had led. Meanwhile during her investigation, Martha is taken captive by more Sontarans and is herself unwillingly cloned in order to allow them to gain access to UNIT. After narrowly escaping death by confusing a UNIT vehicle equipped with ATMOS with UNIT solider Ross,the doctor seeks out the help of Donna at her families home. After Donna introduces the doctor to her mother Sylvia and grandpa Wilf, he attempts to examine the ATMOS device installed in Sylvia's car-the reason Donna claimed to leave the doctor behind in the first place. Upon finding it emitting a poisonous gas,Wilf locks himself in the gas filled car trying to repair the damage when the doctor realizes every ATMOS fitted car in the country experiencing the same gas emission.
This episode not only provides a new chance for two more doctor companions to meet, but sets a stage for a story that allows them to understand their relationship to the doctor. In contrast to Rose Tyler earlier, Donna and Martha click immediately as a female buddy team. It also provides an opportunity to do a modern UNIT story,with the organization being a far more mobilized setup than it had been in the third doctor/Jon Pertwee era. The sociopolitical commentary of this program is also an important feature as well. Luke Rattigan is depicted as a brainy but stereotypically cocky young adult-with an insecurity and need for power than keeps him from being fully aware of the scope of the Sontaran's plans to murder humanity wholesale. Of course the story is also not shy about pointing out the irony of ATMOS technology,intended as a green alternative to fossil fuel, is based on alien technology intended to kill human beings.