SG-1 returns from a mission, and needs to report to the infirmary to see if they brought back any illnesses. Meanwhile they hear there's been a chemical spill somewhere in the complex. Once in the infirmary, every team member needs a precautionary injection, but Dr. Fraiser injects them with a sedative, rendering them unconscious. Teal'c's symbiote protects him though - he wakes up and sees General Hammond and Fraiser in a conversation with a few unknown aliens. Teal'c escapes and manages to wake up Sam. Sam makes her way out of the complex unseen, while Teal'c distracts the enemy. Once outside, Sam calls Colonel Maybourne, which she meets in a cafeteria. He is accompanied by Jack and Daniel, who explain Sam that she has been exposed to the chemical spill's gas and has been hallucinating since. Has she really, or are there indeed aliens impersonating everyone in the SGC?