After a birthday kiss from her earthling boyfriend Dominic, Dr. Janet Fraiser's Goa'uld-engineered adoptive alien teen daughter Cassandra collapses and is diagnosed with a long dormant retrovirus. Daniel discovers video footage made by SG-1 showing that teenagers on her home planet Hanta commonly get a similar 'brain-fire', which is mysteriously cured after a ritual stay in the woods. SG-1 travels there and discovers the lab of the now imprisoned Nirrti, who must have cured them as part of a genetic experiment to produce ideal hosts, of which Cassandra represents a new stage. Then Cassandra's virus causes a transformation, and a Goa'uld is detected in SGC- it's the escaped Nirrti, who offers to trade their lives.
—KGF Vissers