“The Enemy Within” is episode three of season one of Stargate SG-1.
At the end of the previous episode “Children of the Gods: Part 2” the team had successfully returned to Earth with Teal’c, Apophis’ former First Prime who had helped them escape, as he did not believe that the Goa’uld are the gods that they claim to be, and some rescued humans, but without Daniel’s wife. Kawalsky, just before the went through the stargate, had been attacked and infected by a Goa’uld larva.
The Goa’uld are launching attacks on Stargate Command, making it difficult to gate out, as the iris protecting the gate has to remain closed, and Teal’c is not trusted enough to be allowed on SG-1. During this time, the larval Goa’uld tries to take control of Kawalsky. Things get a bit tense.