Season 1 Episode 03 - The Enemy Within

Apophis continues to bombard the Stargate with bombs - fortunately, the newly installed titanium iris prevents them from materializing on this side of the wormhole. At the base, Jack wants to add Teal'c to SG-1 but the government doesn't trust him and has sent military intelligence officer Colonel Kennedy to supervise Teal'c "debriefing."

Kawalsky is reporting headaches, unaware (as is everyone else) that a Goa'uld symbiote leaped into his body during combat (in "Children of the Gods"). Under the symbiote's control, Kawalsky tries to leave but is stopped. Hammond orders risky surgery to try to remove the Gou'ald.

Teal'c provides assistance to the doctors in removing the alien, and they apparently succeed. Despite his assistance, Kennedy insists on transferring Teal'c to a separate facility for "experimentation." However, "Kawalsky" goes on a rampage as it becomes clear they didn't get it out in time. The symbiote attempts to set the base's self-destruct then head for the Stargate, but Teal'c intercepts him. As Jack and Hammond try to override the self-destruct, Teal'c struggles with "Kawalsky" and shoves his head partially into the active Stargate. When the gate shuts down, the symbiote is destroyed and Kawalsky killed. Having proven his loyalty, Teal'c is allowed to stay on the base and becomes the fourth member of SG-1.