SG-1 arrives at PX3-989 to find a manufactured chamber with lights but no occupants. The room is part of a larger compound which they explore. They find a computer which Sam activates, but they're stunned by a burst of light. Upon awakening, they are dressed in black clothing and their equipment is gone. They are greeted by a friendly humanoid alien, Harlan, who apologizes for their loss of equipment and uniforms and for the fact he had to stun them. Harlan introduces himself as the last surviving Altarian and notes that the planet's surface is highly radioactive.
SG-1 begins to notice that they have . . . improved, with minds capable of calculating figures rapidly. When Harlan doesn't provide any answers, they take their weapons and head for the Gate and Harlan warns them it's not safe to leave, but they ignore him and go back. But when Dr. Fraiser tests them she determines that they have no heartbeat, Teal'c is missing his symbiote, and their blood is now milky-white. Jack peels back his arm's skin to reveal . . . circuitry. Hammond confines them but it's clear that SG-1 retains all of their memories and personality.
Hammond prepares to send SG-5 to the planet but SG-1 suggests they go since otherwise Harlan will do the same thing to SG-5. It becomes clear that SG-1 is weakening rapidly and Hammond has no choice but to send them through. They recover on the other side and Harlan informs them that the transferal is permanent and that he needs them to help him keep up with repairs. Teal'c leaves on his own, apparently affected by the loss of his symbiote. Harlan reveals that all of his people transferred their consciousness into robot bodies and then either killed themselves or left through the Stargate to explore and never returned, leaving him to take care of the place.
A coolant leak threatens the compound and SG-1 is forced to help Harlan repair the systems. As Jack tries to open the vents and relieve the pressure on the system, Teal'c shows up to attack him. Harlan is forced to destroy him but then goes off to make a new Teal'c. The others follow him and realize that for Harlan to create a new robot he must still have the originals. The robots wake up their organic originals and nobody is particularly happy at the circumstances. The originals and copies hash things out – the robots agree to stay behind (as they don't really have a choice) and bury the Gate, and the real SG-1 departs.