At Stargate Command Colonel O'Neil arrives at the infirmary and is stunned at seeing the civillian Doctor Carter, who is in turn is amazed to find Jack alive. Major Kawalsky also appears and Frasier explains that Doctor Carter and Kawalsky have passed dental checks to confirm their identities. Kawalsky explains they are from an alternate version of space-time, having fled here after the Goa'uld seized control of Earth. Doctor Carter explains they cannot be followed as they brought the control to the 'quantum mirror' with them.

SG-1 reviews the tape of Doctor Carter being interrogated at Area 51 and insisting she is a member of the 'SGA' and was the person who first made the Stargate work. A stunned Major Carter, although finding an alternate version of herself "too weird", confirms the possibility. When Doctor Carter and Kawalsky join the briefing, General Hammond explains they have encountered cross-reality travel before (There but for the Grace of God) and were able to prevent the Goa'uld destroying Earth because of the information learned from this experience. Doctor Carter explains that their Beta Site's Stargate has been buried and their Earth enslaved, leaving herself and Kawalsky no choice but to enter a different reality through the quantum mirror. If possible, they would like to stay in this reality.
Major Carter observes that incidents such as Teal'c's defection and her joining the military have contributed to this reality's survival, although Doctor Carter takes some offence as she can't see herself in the military. After debriefing the major apologises to her alternate self for the unintended offence but Doctor Carter assures none is taken; SGA knew the Goa'uld were coming for months yet were unable to find a way to stop them.

Asylum for the two refugees is granted surprisingly swiftly and General Hammond is encouraged to find positions for them at the SGC. When Colonel O'Neil informs Doctor Carter of the news, he finds her crying over a wedding photo of herself and Jack. Doctor Carter and Colonel O'Neil were married in the alternate world, she explains, and their first anniversary dinner was interrupted by the Goa'uld invasion. Samantha witnessed Jack's death during the invasion days earlier and now she has come to a world where her husband barely recognises her.

Alone in her barracks after being consoled by O'Neil, Doctor Carter goes into an apparent seizure, her face severely distorted. Frasier is unable to make a diagnosis but the Carters realise the 'sickness' is actually entropic cascade failure resulting from the two of them existing at the same time, an effect Doctor Carter did not expect to happen so soon. Major Kawalsky is unaffected since he no longer exists in this reality but Doctor Carter's condition will gradually deteriorate. The only solution is to return Carter to her own world but doing so will leave her at the mercy of the Goa'uld.

Daniel Jackson proposes SG-1 escort Doctor Carter and Major Kawalsky back to their reality and help fight the Goa'uld from their Earth. SGC's computer still holds the destination chevrons for the Asgard's home planet (The Fifth Race) and if the alien power generator can be reassembled the group can travel to Thalla and raise the Asgard. Major Carter remains behind but Teal'c goes to provide the group cover, expecting return or death before he develops seizures. This reality's quantum mirror will be destroyed when SG-1 returns.
While the Carters reconstruct the power generator, Major Kawalsky shows Jackson how to operate the quantum mirror: if left on the mirror is connected to the same reality but loses its place when turned off. Hence, when Doctor Carter turned the mirror off it lost the connection to Kawalsky's reality and the two must 'reality-surf' to find the correct destination through trial and error.

Entering the alternate reality, Jackson is forced to deactivate the quantum mirror to avoid a Jaffa patrol. O'Neil and Kawalsky follow the patrol, finding a platoon assembled to welcome a bearded Apophis through the Stargate. When the platoon is dismissed, however, First Prime Teal'c realises the humans' presence and summons the Jaffa. The rogue Teal'c kills his alternate self and misleads the Jaffa by taking Doctor Carter as his prisoner. O'Neil and Kawalsky depart to install the power generator while Daniel reality-surfs home.
Apophis tortures General Hammond in an effort to learn the destination chevrons for the Beta Site but fails, ordering Teal'c to beat the information out of either Hammond or Carter. Teal'c orders the guards out of the room, however, to explain to the General the successful mission through the quantum mirror and their plan to contact the Asgard.
O'Neil and Kawalsky successfully install the power generator and return to the mirror, only to find the Jaffa have discovered Daniel and the three are captured. Doctor Carter is able to install the dialling program and reach Thalla but Teal'c's cover is blown in the process. Apophis demands to know how O'Neil could be alive without a sarcophagus but when Jackson tries to explain cross-reality travel the System Lord kills General Hammond. Apophis prepares to use his ribbon on the remaining captives but one Jaffa warns of an incoming ship. The Jaffa flee as Apophis, his guards and Hammond's body are caught in teleportation beams. Doctor Carter and a resurrected General Hammond are returned to the control room, the former explaining she reached the Asgard.

With the correct reality found, Colonel O'Neil, Daniel Jackson and Teal'c prepare to return to home. Doctor Carter laments having to farewell her husband for the second time, prompting O'Neil to ask how she could possibly marry such a loser. On the spur of the moment Doctor Carter gives the colonel a long farewell kiss, pulling away when she realises it is not with her husband. SG-1 return through the quantum mirror, the device closing on a weeping Doctor Carter.

In the words of Colonel O'Neil, the average IQ of the world has just gone up with the addition of an alternate Samantha Carter. It's a bit of a humanity thing, with the main characters forced to confront life when the sorely missed Charles Kawalsky returns to be alive and well.

The addition of Dr Carter being unable to survive in this reality is added just so we can't enjoy an alternate Carter throughout the series, but the special effects used to show the effects on the good doctor are a bit over the top; I hardly think they were necessary, since they added to an unnecessary gross-out factor.

The sequence where most of SG-1 travel to Dr Carter's reality was a pretty good twist, and this episode is one of the best of Season Three.

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"Anyone who seeks fame need only become familiar with all I have acheived." - Claudius Galen
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