In the Line of Duty (S02E02)

Col. Jack O'Neill and the SG-1 team are on a mission to Nasya, hoping to rescue the survivors of a planet that has been attacked by the Goa'uld. During the rescue, however, Capt. Samantha Carter's body is taken over by a Goa'uld that was hiding in the body of a mortally injured Nasyan. Carter's fellow team-members don't notice the change, but when the team returns to Earth, Carter's young friend Cassandra recognizes the Goa'uld immediately. Cassandra tells O'Neill, who orders Carter confined to a cell. O'Neill bargains with the Goa'uld for Carter's life. The Goa'uld tells Teal'c that he is Jolinar of Malkshur, one of the Tok'ra Goa'uld, a rebel group opposed to the bloodthirsty ways of the system lord. However, Jolinar also reveals that he has been followed here by Ashrak, an assassin trained in the ways of the Goa'uld. Unless the SG-1 team can stop him, Ashrak will kill Jolinar, taking Dr. Carter's life as well. (Metro-Goldwyn-Mayer (MGM))