The only thing really important about this storyline is that SG-1 ends up finding out about a race called the Tokra, who although having symbiotes themselves, rebeled from the Goa'uld opressors long ago, and are now a rebel faction run by a council. The jist of how it happens is that when SG-1 is evacuating a world that the Goa'uld are attacking, while trying to save a man, Sam is infested with a symbiote without her knowledge or consent. When she get's back to the SGC nothing seems unusual aside from a little sore throat, and her personality appears to have changed some what.(like when she punches Jack's arm affectionaly) They soon learn that she has one in her when, when she visited Cassie, Cassie detected the symbiote inside of her, and as the symbiote takes over Sam's body, she attempts to escape Earth through the Stargate, but is stopped in her attempt.
Later while she is in her holding cell they find out that a Hashrak, or Goa'uld assassin on a mission sent by the system lords, was sent to kill Jolinar, the Tokra inside of Sam. He nearly kills Sam, but thankfully Jack gets there in time and brings her to the infirmary. She is about to die when Jolinar gives it's life to save her. They catch and kill the Hashrak. But Sam is clearly emotionaly scarred. And it seems the only one who can get really close to her right now is Cassie. A very good episode, Amanda Tapping really showed her caliber here. This episode is and will remain one of my favorite episodes of all time.