Its Cassandra's birthday and she wants to go out with a boy, Dominic. Janet tells her that she can go out after she has had some of the cake Sam has made her. Cassandra storms off outside to see Dominic and he gives her a birthday present. They kiss and the light in the porch explodes, lights in the house also dim. Cassandra collapses.

Sam and Frasier rush her to the infirmary, they find she is infected with some retrovirus. Cassandra deliriously says she can't finish the harvest and needs to go into the forest. In a briefing Daniel tells them that SG-1 saw a few children act like this when they were on Hanka, the locals said they had the 'mindfire' and were touches by the Gods. They were then sent out into the woods and came back later cured.

SG-1 travel to Hanka and start to look around the forest near the village. They find a tree with a luminous hand print on it. Daniel touches the print and a fire lights, Jack and Teal'c go to look at it and are ringed to a lab underground. They decide it must belong to the Goa'uld Nyerti who was apparently experimenting on the local children. They find some samples and decide they will take some back to the SGC.

At the SGC Frasier finds Cassy has left her bed, she goes to look for her but Cassandra shouts at her saying that she has to leave, she has to be physically restrained by soldiers. Sam returns to the SGC and sees Cassy, she tells her about the lab and what they believe Nyerti was trying to do.

At the lab Jack, Daniel and Teal'c continue to look around and take samples. As Daniel fiddles near a console the rings activate, he says it must have been him. As they leave something watches them. Back at the base Sam goes to play chess with Cassy as it is Saturday. Cassy can move the pieces with her mind. In a briefing they discuss the situation, Daniel says he has translated Nyerti's notes further and that she was trying to advance the evolution of the Hankans to create a advanced human for a host. Frasier goes to see Cassy and tells her to fight it, Cassy says she wants to let it happen. In another room Sam and Frasier look over some test results, after they have left something else accesses the computer.

A while later Frasier is studying some files, she leaves the room and Nyerti appears, she looks at the files but disappears before Frasier returns. Sam and Frasier talk but the alarm goes off, the rush to Cassy's room, she tells them a Goa'uld was there. They call out the zats and TER's and go hunting for who they think it is, Nyerti. Jack says with Cassy and Nyerti soon comes back, Jack manages to zat her.

Nyerti is held in a room and Hammond & Jack go to see her, she says the end is near for Cassy. Nyerti offers to cure Cassy for her freedom, invisibility device and a sample of Cassy's blood. Hammond decides to think it over. In the infirmary Cassy's condition is getting much worse, Frasier leaves and goes to the holding room, she pulls a gun on Nyerti. Hammond arrives and tells Frasier to stand down, he has decided to agree to Nyerti's freedom (But not the invisibility device and blood sample) in exchange for saving Cassy.

Nyerti manages to cure Cassandra and Jack them escorts her to the gate room. As she leaves Nyerti tells him that she wouldn't have done the same in their position.