Last of the Time Lords
"The Sontaran Stratagem" features the first appearance of the alien Sontarans to the series since the 1985 Colin Baker story "The Two Doctors", as well as former companion Martha Jones (Freema Agyeman), last seen in "Last of the Time Lords." The episode takes place on present-day Earth, where Martha and UNIT summon the Doctor (David Tennant) for assistance concerning ATMOS, a revolutionary piece of green technology installed in 400 million cars worldwide. ATMOS is later revealed to be part of a Sontaran plot to poison the atmosphere.