The SG1 team arrives at a new planet and find a medieval Christian village, there they find a girl, Mary, shackled to a post in the middle of the village. They unshackle her and take her to the friar's home, he explains to the team that she was chosen by the village Canon to be sacrificed since it is believed that Mary is possessed. At first the friar thinks the SG team are demons, but they assure him they are humans and just want to help. Jack says Mary is just sick, and that they can help cure her, but the friar says the elders will perform a trepanning ritual that night to cleanse her before she is sacrificed.
At that moment they hear a horn echoing over the village, the friar says is the demon coming. Jack takes a look at the window, and sees that what the villagers call demon is merely a Unas, the first Goa'uld host. The Unas says that his lord Satan calls for five humans to be placed in the town center, and at sunrise he will be back to collect them. The team realizes that Sokar is the one behind this, posing as Satan and demanding sacrifices so he can gather hosts.
They finally convince the friar that they are not demons, when the Canon arrives, he doesn't believe they are friends, and think Teal'c has the mark of the devil, because of his Jaffa tattoo. The Canon says they will be struck down by God, and a lightning bolt comes from the sky and struck all the SG team. They wake up in a holding cell, minus Teal'c. The friar tells them that he tried to convince the Canon that they are not demons, but he refused to believe, and the Teal'c must endure the tests. Daniel says that in medieval times there were a number of tests to prove if a person was possessed by a demon.
The Cannon first puts a heated iron piker in Teal'c's forehead, to sees if he feels pain and if he doesn't it means it really is the mark of the devil. Teal'c doesn't feel pain and they proceed to the next test, submerging him in the water, if he floats, it means he really is guilty of witchcraft and should be burned to the stake. They throw Teal'c in the water and he sinks, they wait for a while but he doesn't come back.
Teal'c's body is being prepared to be buried, and the Canon says they can take Teal'c with them through the Stargate if they wish. Jack asks for their gear, and Simon the friar says he will return their gear if they help him and Mary. The Canon calls Simon for the trepanning ritual, and Daniel convinces Jack and Sam to try and stop it before Mary is killed. The team decides to take Simon and Mary with them to Earth. At that moment, Teal'c wakes up, and the Canon changes his mind and decides to sacrifice them all, including Mary.
At dawn, they ask for Simon's help but he is too frightened to do anything. The Unas arrives and takes his sacrifice with him. On the way to the Stargate, they manage to escape the Unas and run through the forest. The Unas find them and Simon shows up, with the SG gear and Teal'c's staff, Jack urges him to shoot the Unas, but after three blasts, the Unas caches up with Simon and disarm him. Mary reaches for the staff and Teal'c frees them. Sam Daniel and Mary head for the gate while Jack and Teal'c go after the Unas, to save Simon.
The Canon finds Simon and the Unas, and when jack and Teal'c arrive, they find the Unas dead and Simon unconscious. The Canon says the Unas hit Simon and then collapsed on the floor, dead. He thanks them, and Jack asks for the ring that controls the God smiting ray. Simon wakes up and they head to the gate. Carter feels the presence of the Goa'uld inside the Canon; it went from the dying Unas into him. The Canon's eyes glow and he tries to attack them, but Jack shoots him. Sam dials home, and Jack tells Simon to bury the gate after they are gone. Simon says he will do that and there will be no more sacrifices in their village.