During an evacuation Nassya, a planet being attacked by Death Gliders, Carter finds an unconscious man and performs CPR. However, when she does mouth to mouth, the man transfers a symbiote into her! The man dies as Jack comes to Carter, and the two run through the gate.
At the briefing, Hammond and SG-1 are puzzled as to why the Goa'uld would attack a civilization with such a low level of technology. Hammond informs them that there are too many injured survivors for the base to hold, so some are being sent to the USAF hospital and also that SG-1 needs to find a planet for relocation. After the briefing ends, Carter goes to get checked by Dr. Fraiser. She notices small abrasions in Carter's throat but just thinks that it's due to a sore throat. The next day, while Fraiser is checking on patients at a hospital, Carter visits Cassandra. However, when they hug, Cassandra senses the symbiote.
Back at the SGC, Jack gets a call from Dr. Fraiser, who tells him to come to the hospital because Cassandra needs to talk to him. Cassandra tells Jack that Sam is now a Goa'uld and that she threatened to kill her if she told anyone. Jack and Fraiser then rush back to the SGC and get there right before SG-1's departure. Carter is injected with tranquilizers by Jack, but she is unaffected! She grabs a gun and orders that the gate be opened, but she is shot with more tranquilizers. After Jack grabs her gun, she pulls out a grenade but collapses. Later, Dr. Fraiser tells the others that the symbiote entered Carter through the mouth, and Jack realizes that the symbiote was from the man she gave CPR to. General Hammond then orders Jack to interrogate the Goa'uld.
At the hospital, Daniel talks to the wife of the man the symbiote came from. Afterwards, he figures out that the symbiote was in the man for months, but he is puzzled that the man never showed any aggression or changes in behavior. Elsewhere in the hospital, a doctor goes to check on one of the survivors of the attack, but is then attacked by the patient.
The patient removes the bandages for his burns and is fully healed. He then goes to the sink and spits up a sort of Goa'uld hand device. Sometime later, Dr. Fraiser goes to check on the patient, but it is instead the doctor wrapped in bandages!
At the SGC, Jack talks to the Goa'uld, who says that if it is allowed to go through the gate, then it will leave Carter and find a new host. Jack, however, doesn't believe it. Teal'c later visits the Goa'uld, and it claims to be Jolinar of the Tok'Ra, enemies of the Goa'uld! At the hospital, the patient, now dressed as a doctor, is going around scanning other survivors with the hand device. Daniel sees him but doesn't suspect anything. Meanwhile, after seeing blood results for the patient, Dr. Fraiser runs to the patient's room but finds the doctor instead.
When Jolinar is told about the patient suddenly disappearing, she tells Jack and Teal'c that it is an Ashrak, or hunter, that has been after her. Meanwhile, unable to find Jolinar in any of the survivors, the Ashrak uses his hand device to brainwash a military truck driver, allowing him to go to the SGC with the survivors. As the Ashrak locates Jolinar, Jack and Teal'c are told that the driver reported to the infirmary claiming to have blacked out. However, by the time they make it to there, the Ashrak has already done its job and has left. Carter is rushed to the infirmary as the Ashrak heads to the gateroom, but the gate shuts down before he can get to it. When Daniel recognizes him as the doctor, he holds him hostage and orders that the gate be opened. However, Teal'c, using a zat gun, shoots the two. When Daniel is let go, Teal'c fires again, killing the Ashrak. Meanwhile, Jolinar's life signs are getting weaker, causing Sam's to drop, too. The symbiote then dies, but Sam's life signs return to normal. When she wakes up, Carter weakly tells Jack that Jolinar gave her life to save her.