Daniel Jackson, Samantha Carter and Teal'c rush for the Stargate, and quickly dial back to Earth. They are under fire from a group of Jaffa soldiers, and barely make it back alive. Colonel Jack O'Neill and Lieutenant Tyler remain on P7S-441, pinned down by enemy fire.

But when they request that General Hammond send them back with reinforcements, he denies their request and orders them to the infirmary for a complete evaluation. For reasons they can barely speculate on, no one else at the S.G.C. knows who Tyler is, or remembers the last three weeks he has spent training with SG-1.

Back on the planet, O'Neill and Tyler find a small, stone structure to hide out in. Jack's mind is focused on his training, and with weapon in hand he keeps a lookout for the Jaffa. Tyler is injured, and can't stand up – let alone help in their defense.

Carter, Teal'c and Jackson try to find some answers, though Hammond has their access to the base restricted. Sam hacks into the base computer system, and to her surprise finds no record of a Lieutenant Tyler having ever served at the S.G.C. But she makes another discovery: an anonymous user, identified only by a four-digit code (User 4574), is accessing the computer records from outside the base – including SG-1's personnel files.

As the day begins to wane, O'Neill concludes that reinforcements aren't coming right away. He digs in, setting explosive charges around the perimeter of their camp. Tyler is appreciative of what O'Neill is doing, though he seems to feel guilty for having put him in this situation. If reinforcements don't show up soon, Jack isn't sure how long he can hold off the enemy army by himself.

Colonel Frank Simmons visits the S.G.C. from the Pentagon to conduct an investigation into possible security problems. He interviews Sam, Daniel and Teal'c individually, despite Hammond's objections. The general makes some calls to his government connections, but can find out nothing about Simmons' orders or motives – but suspects he may be involved with the N.I.D., a secret government organization that has tried to take control of the S.G.C. in one form or another since before the inception of the Stargate program ("Touchstone"). Even the president will not intervene now.

Simmons confronts Daniel about his motivations, and tells him outright that he doesn't think a civilian archaeologist with emotional ties to the mission has any business in a field unit. He questions Teal'c's true loyalties, of course, given the Jaffa's recent reversion back to Apophis' service ("Enemies," "Threshold").

And Carter? She isn't trustworthy either, according to Simmons. In addition to everything else she has been through in the last few years, the last time they met she harbored a powerful alien in her home ("Ascension").

But Carter knows a thing or two herself, and confronts Simmons about accessing the base computer records externally. She knows his anonymous user I.D.

Dr. Janet Fraiser soon discovers the reason why SG-1 has had their memories altered. She finds a powder residue on Teal'c's jacket that causes her to see a brief flash of Lieutenant Tyler, and analyzes it. The chemical affects those parts of the brain dealing with facial recognition and memory, and can be used to make a total stranger believe that he or she knows the chemical's user personally.

With the Jaffa army pressing in, Tyler confesses this to O'Neill. He reveals his true, alien identity, telling Jack that his species (the Reole) uses the natural chameleon ability to survive. The Goa'uld have learned of this, and hunt them to obtain the power of memory alteration. He does not want O'Neill to risk his life at his defense.

But Jack engages the Jaffa in battle, determined to protect Tyler – no matter who he really is. Just when they are about to be overwhelmed, reinforcements arrive. The rest of SG-1 has been sent to recover them.

Tyler does not want the team to risk their lives for him any longer, and flees into the woods, trying to lead the Jaffa away from the gate. He is quickly captured. SG-1 recovers the cannon from a death glider downed by Colonel O'Neill, though, and use it as extra firepower to rescue Tyler and clear the Jaffa army away from the Stargate.

The alien says goodbye to the team and returns to his world, where he says they will bury their Stargate in an attempt to hide from the Goa'uld. SG-1 returns home, but finds that Colonel Simmons plans to report serious concerns about the S.G.C., threatening to take down General Hammond's command.