Ep 14. Foothold

SG-1 returns to Earth to find that the Stargate has been relocated to a new facility run by the NID. General Hammond assigns SG-1 to investigate a supposed alien abduction of a high school classmate of Cassandra Fraiser. SG-1 infiltrates the facility and discovers that the NID has been taken over by an alien race called the Talthuns who have taken on human form. The Talthuns are implanting the minds of the base personnel with devices that allow them to control their actions and gain access to sensitive information. Through their investigation, SG-1 is able to expose the Talthuns and prevent them from using Earth as a foothold for their invasion.