Daniel and Vala travel to Atlantis in search of Merlin's anti-Ori weapon, while Sam and Mitchell join forces with Dr. McKay to lock out the Ori supergate.

While assisting SG-3 on an offworld mission, Teal'c encounters a savage alien creature that is killing the inhabitants. SG-1 investigates and learns that a parasite is to blame. The team suspects that the Ori are responsible, but they are shocked when they find out the real cause.