SG-1 travels to P3X-562 and discovers a valley full of broken crystals. When the others are elsewhere, Jack finds a whole crystal and examines it - he gets an energy blast for his troubles and drops unconscious. The crystal then creates a duplicate O'Neill and it goes back through the Stargate with the rest of SG-1.
After debriefing, "Jack" goes to see O'Neill's estranged wife, Sara, and asks for their son, Charlie (who died several years earlier after accidentally shooting himself with Jack's gun). Back on the planet, Jack wakes up and comes back through the Stargate. Sam and Daniel determine the crystals are sentient creatures - the containment for energy beings that can read minds and mimic others. They set up a dialogue with the energy creatures, who relate how the Goa'uld considered them a threat and destroyed many of them.
It becomes clear that the energy beings can't survive for long in Earth's relatively high electromagnetic field. The "Jack" duplicate begins to grow unstable, putting Jack's wife and his surroundings at great risk. SG-1 tracks the duplicate down to the hospital emergency room, and it apologizes - it never intended to harm Jack. As a final gesture it takes on the form of Charlie so that Jack and Sara can take the opportunity for a final goodbye with their "son." The energy creature then departs back through the Stargate before it can explode.