When Jack is injured unintentionally by a crystal alien, the alien duplicates Jack's form and out of fear of retribution tries to heal Jack. He returns to Earth in place of Jack, while Jack is still unconscious on the planet, to find the one thing he feels will accomplish this; the one thing that cannot be found again. Meanwhile, the 'real' Jack has returned and is trying to convince SGC that he is who he says he is.

Having regained control of the X-303, the SG-1 team finds itself in an unknown part of space. Rescue comes in the form of the Asgard, specifically Thor, who needs their help. The Asgard home world has been overrun by the Replicators and they require SG-1's help as well as the relatively low tech X-303. The Asgard had set a trap for the Replicators and hoped to use a time dilation device that would give them the time they need to find a way to defeat them. The problem is that the Replicators have turned off the device and the Asgard want SG-1 to go in and activate it. They accept the assignment but to meet their objective, O'Neill will face a moral dilemma.