Having spent their last mission looking without success for clues to where Daniel's child by Sha're might be, the SG-1 team find themselves rushed into the infirmary for their routine examinations.

However, this time the examinations are anything but routine, as Dr. Janet Fraiser gives each member of the squad an injection that makes them lose consciousness. One by one, our heroes pass out. Fraiser then begins to perform some sort of testing procedure.

Teal'C applies force
Teal'C wakes up first because, as he soon learns from eavesdropping on the doctor and General Hammond, his extraterrestrial physiology made him harder to sedate. Feigning sleep, he allows strange alien guards to take him to level 16 of the Cheyenne Mountain base, where he promptly assaults the guards and escapes.

The two of them then sneak into the communications center, from which they are able to observe General Hammond parlaying with aliens who are coming freely through the Stargate. Unfortunately, the aliens soon realize that two of their prisoners have escaped, and alarms go off. Teal'C fends off the troops and is eventually overcome by a gas weapon while Carter flees the complex to call General Maybourne, whom she warns that the aliens have established a "foothold situation" inside Stargate Command.

Not a happy man
When Teal'C next regains consciousness, he is back in the infirmary, but this time his captors have put him in restraints. Dr. Fraiser explains to him that the chemical spill affected his mind, making him attack some SGC troops while in a delusional state. He appears to accept the outlandish story, but refuses to tell the doctor anything about where Carter has gone.

Meanwhile, Carter meets with Maybourne, but Hammond has gotten to him first. Maybourne repeats the "delusional" story and seems convinced that the chemical spill has made Carter unreliable enough that she should be restrained.

However, O'Neill and Jackson show up and, although they also vouch for the chemical spill explanation, refuse to let Maybourne put handcuffs on their teammate. Instead, they convince her to go back to SGC peacefully.

During the plane ride back to the base, "O'Neill" transforms into an alien in full view of Samantha, considerably affecting her decision to go back without a struggle. She grabs Maybourne's pistol and "Jack" and a member of the flight crew are shot in the struggle that follows. Both bleed blue.

Maybourne cuts himself, bleeding red, to prove to Carter that he is in fact human, then sends her to check out the flight cabin for more aliens.

Copycat invasion
In a dark, spooky room, various military personnel including the members of SG-1 are hanging from the ceiling suspended by slimy glowing cables apparently asleep. Jack wakes up and looks around, to be joined soon afterward by a fellow who looks exactly like "Davis," the crewman Carter shot on the plane.

Dr. Fraiser wheels in a gurney carrying another soldier. She attaches him to his own set of slimy cables, then one of her alien guards transforms itself to look like the soldier. They all leave.

Back on the plane, Samantha takes a slimy module from the dead body. She opens Daniel's shirt and finds the same device on him. When she takes it off him, she finds that it gives her the ability to look like Daniel in every respect. It also reveals that, without benefit of the device, "Daniel" is another of the aliens, whom Maybourne promptly shoots.

In the infirmary, General Hammond walks in on Dr. Fraiser while she is performing some sort of torturous experimentation on Teal'C's Goa'uld symbiote. The general warns her that Carter is on the way back to the base.

Meanwhile, the aliens have trouble from a different vector as Jack and Davis -- for this is really him, not the alien impersonator Carter killed on the plane -- break free of the alien restraint gear. Finding Fraiser in the mass of cables, O'Neill tries to break her free as well, but the headgear only gives off a strange noise so he reattaches it.

"Fraiser" herself, likely disturbed by this interference with her human doppelganger, wanders in to investigate but the two men easily subdue her.

It ends in chaos
Carter sneaks back into the base. We next see O'Neill and Davis threatened by "Daniel," who is of course actually Samantha wearing the illusion device. She fills them in on her discoveries, including the fact that the original "Jack" masquerade was disrupted by the engine noise of the plane. This has prompted her to create a device that broadcasts a specific tone that interferes with the illusory disguises.

Masquerading as Daniel again, Carter gains access to the lab and records the tone while Jack frees Teal'C in his usual artful fashion.

An abortive fight scene emerges in the lab when General Hammond interrupts Carter's work and knocks her down just as she is preparing to broadcast the tone to the entire base. However, he gets an apparently urgent page and loses interest in the fight, wandering off.

Now unmolested, Carter turns on the intercom and reveals all the aliens for the craven blue-bleeding monsters they are. The aliens promptly lose their nerve and start to run back to the Stargate, taking the knowledge they have gained from SGC with them.

Luckily, Maybourne's troops arrive and Carter closes the Stargate, trapping most of the aliens on Earth. In a particularly inscrutable gesture, the alien leader blows himself and his troops to hell, leaving no prisoners or hostages.

To prevent anything like this from happening again, the team vow to broadcast the tone on every Stargate opening. Maybourne offers his compliments and goes back to Washington.