Episode 24 of "Stargate-SG1"
s02e02 In The Line Of Duty

As SG-1 help the inhabitants of the planet Nassya flee through the stargate from a Goa'uld attack, Carter becomes possessed by a Goa'uld (unknowingly however). Once they get back to Earth Carter tries to act as normal as possible, however she begins to act strangely and goes to see Dr. Fraiser. Cassandra comes to see Sam and Cassandra senses that Sam has a Goa'uld inside of her. Meanwhile as assassin is amongst the refugees brought back from the planet Nassya. He has been dispatched by the Sysytem Lords to kill the Goa'uld symbiant which is inside Sam. Once it is revealed that Sam has a Goa'uld in her head, she is locked up behind bars. The Goa'uld them reveals itself to be Jolinar of Malkshur, and demands release. Jolinar reveals that she is a member of the Tok'ra, a band of rebel Goa'uld, who oppose the System Lords and seek power to do good. Teal'c has heard of these Tok'ra and is convinced. The assassin infiltrates SG Command and tortures Jolinar. Teal'c comes to rescue and kills the assassin. Meanwhile, Carter is fighting for her life in the infirmary. When she wakes, she tells O'Neill that the only reason she recovered was because Jolinar gave her life to save Sam's.