SG-5's Lieutenant Barber commits suicide by jumping into the Stargate shortly after a mission on a planet where a palace abandoned by the Goa'uld about 200 years ago is being studied by Daniel. Soon after, Daniel develops violent mood swings and is committed to a hospital with a neurotransmitter brain condition, presumed fatal after all the other SG-5 members die. Meanwhile, SG-3 has visited the planet, finding only the light installation Daniel reported as particularly fascinating and a human teenage boy, Loran, who hid from Daniel's expedition, but now tells Jack he was left there by his parents, scientific researchers. Back in SGC, Jack also develops symptoms. Sam and Dr. Fraiser conclude that it must be some addiction linked to the light, so when Daniel is nearly terminal Jack, who might not have been exposed too long himself, must bring him back. Jack insists they try turning off the light, and leans on Loran, who is looking for a father-figure; he even gives Teal'c an early "birthday present," but has a dark secret.

The Tok'ra are forced to evacuate their newly established base under attack from the Goa'uld. They flee to the SGC's Alpha site. Tensions rise between the allied factions, the Free Jaffa, Tau'ri, and Tok'ra. Jacob and Selmak feel the end of the Tok'ra is near and Sam finds a Naquadah generator has been sabotaged to function as a bomb. Jack suspects the recently arrived Tok'ra and attempts to weed out the traitor.