The Stargate base is put in deadly peril when it is contaminated with a dangerous infection which causes its victims to become mindlessly animalistic brutes.

SG-1 goes to the planet Cimmeria in search of allies against the Goa'uld. Upon arrival, Jack and Teal'c are trapped in a labyrinth, where the only exit is through Thor's Hammer, a device to destroy Goa'uld, but preserve the host. Daniel Jackson must destroy the device that could someday have saved his wife in order to free his friends.

SG-1 inadvertently help a desperate criminal and are charged and sentenced as accessories to his crime. They are banished to a desolate, brutal penal colony and strike a deal with a powerful female prisoner to help them escape. But is this prisoner hiding something from them?

O'Neill and the team return to Cimmeria, the planet where they destroyed Thor's Hammer to help Teal'c escape. They find that the Goa'uld have invaded and many of the people are dead. Can they help to find Thor's weapons to fight off the invasion before they are all captured or killed?

SG-1 arrives on a planet where a culturally 'medieval Catholic' community lives in fear of an Unas they believe to be a demon, which regularly collects human sacrifices to Satan - in fact hosts for System Lord Sokar's army. When Simon implores SG-1 to spare his beloved Mary and take him instead, they even nurse her chicken pox. Simon hopes they are sent by God to deliver them from the satanic burden, but the canon who rules the village as 'spiritual' leader is faithful to Sokar and fears his authority, which rests on his right to select the 'damned soul' to be sacrificed, has been challenged, so he declares Teal'c a demon and incarcerates all SG-1. Witch-trial-type ordeals see the Jaffa 'proven' a demon and executed by drowning, and Daniel refuses the canon's warning to leave immediately. When Teal'c's symbiote allows a resurrection even Sam hadn't expected, they are all designated possessed and sacrificed, along with Mary. Simon follows the Unas with a staff weapon to liberate them, but he's not alone.

Martouf leads a Tok'ra delegation with crushing news for Carter: her father Jacob/Selmak has been captured by Sokar and imprisoned on Netu, an Egyptian name for hell, on a moon of planet Delmal, where prisoners are sent to suffer eternal torture, and from where no one returns; only Jolinar has ever escaped. SG-1 mounts a rescue operation with Martouf, top-priority being any information Selmak can offer about Sokar's plan to subjugate the other system lords, an unprecedented threat for all humanoids in the galaxy. Teal'c must pilot a Goa'uld ship and keep it in orbit as landing is impossible except by small pod, but first Martouf taps into Jolinar's memory, which is partially blocked by some trauma beyond physical torture. Only when they're down and in the custody of Netu's lord Bynarr, does Sam realize he is the key, in a 'personal' way, but not voluntarily, as she had thought. Sokar learns of their presence from Bynarr and orders the team and craft destroyed, but this also means there must be a way out. Bynarr's prime, Na'onak, has, and is himself, a major surprise.

When Daniel's Unas friend, Chaka, is captured by another world's slavers, SG-1 goes to that world to rescue him.

After Sam receives an emergency call from geneticist Dr. Richard Flemming, the SGC learns that he has been working for Immunotech, a company owned by Adrian Conrad, who implanted himself with a Goa'uld symbiote in order to cure himself. Fleming has now gone missing and General Hammond orders Sam, Jonas and Teal'c to investigate. They head to Oregon where Fleming lived and worked and learn from the local Sheriff that Fleming isn't the first Immunotech worker to disappear. The locals are acting strangely and seem to suffer from memory lapses. What they eventually find is a town full of Goa'uld and NID seemingly aware of everything that is going on.

While investigating the source of an addictive corn-like crop, SG-1 becomes involved in a firefight. They are forced to fall back to the gate, but as Daniel is dialing both the Stargate and the DHD are beamed away.