The episode reveals that the Replicators were created by the android Reese as "toys" that she programmed to protect her when the others on her world attempted to shut her down. As the Replicators multiplied, they escaped her control and destroyed her civilization before moving on. After being brought back to the SGC, Reese creates more Replicators out of available materials, which overrun the base and are only stopped after Colonel Jack O'Neill shoots her. Reese is handed over to the Asgard, who find within her a base command that allows them to summon all the Replicators. They intend to use the command to trap all the Replicators on Halla inside a time dilation field that would give them years to devise a more permanent solution. In , Thor calls upon SG-1 to investigate when the time dilation device fails to activate. They find that the device has been reversed to accelerate the passage of time inside, and that the surface of Halla completely converted into Replicator blocks. They also encounter the human-form Replicators (see below), the new faces of the Replicator threat. SG-1 is able to reactivate the time dilation field by appealing to the humanity of one of the human-forms, Fifth.
In the season 8 premiere , the Asgard have collapsed Halla's sun into a black hole to try and destroy the Replicators. However, the Replicators escape, led by a now-vengeful Fifth. Their invasion of the new Asgard homeworld Orilla is turned away after Thor extracts the schematics for the Replicator disruptor from O'Neill's mind. Fifth also creates a copy of Samantha Carter to serve as his consort. Replicator Carter betrays Fifth and takes over control of the Replicators in , and oversees the full-scale invasion of the Milky Way in . She and her brethren are finally defeated using the Dakara superweapon. The Replicators return in Stargate: The Ark of Truth, where one is created by the Asgard computer core on the Earth ship Odyssey by IOA agent James Marrick. The IOA plans to use the Replicators as a weapon against the Ori; however the Replicator escapes and soon replicates, overrunning the ship. Facing off against the Replicator-controlled Marrick, Mitchell manages to find the kill-switch the IOA built into the Replicators, allowing Carter to deactivate them.
The main Replicator directives are to multiply and to assimilate new technologies. Replicators utilize whatever materials are available; in , the Replicators created from a Russian submarine are made of steel and susceptible to rusting. Multiple blocks are required for replication, and the process requires a large amount of energy. Replicators are attracted to the most advanced technologies available, thus they ignore the relatively crude technology of the Earth ship Prometheus in . Replicators adapt very quickly to new technologies and they are capable of enhancing technology they encounter beyond its original specifications. Replicators are impervious to all known handheld energy weapons, including zat'n'ktels and staff weapons. They can be shattered by projectile firearms, though given time the blocks will re-assemble. Replicator bugs produce a corrosive liquid ("Replicator spray") capable of dissolving through any known material, including the nigh-invulnerable armor of Kull Warriors. In close quarters, they can produce an electrical discharge to kill their opponents. Replicator bugs are strong for their size; a single one held open the hydraulic blast doors of the SGC's gate room in .
At the end of "Unnatural Selection", all the human-form Replicators are trapped on Halla after SG-1 activates the time-dilation device. The only one of the original six Replicators who appears in subsequent episodes is Fifth, who escapes Halla in "New Order".
Another human-form Replicator also appears in "New Order", Eighth. He was played by James Bamford, the stunt-coordinator of Stargate Atlantis, which was shooting its pilot during "New Order"; the producers wanted to make Bamford feel as part of the family. Eighth is recovered, damaged and inactive, by Thor, as a means to tap into the Replicator network and determine the location of Fifth. Eighth activates and breaks free aboard Thor's ship, and is disintegrated by Jack O'Neill using the newly constructed Replicator Disruptor. Eighth's name is not mentioned in the episode, but appears in the ending credits.
Played by Patrick Currie, Fifth is a human-form Replicator introduced in "Unnatural Selection". He is the fifth human-form to be created on the planet Halla, and unlike the others he lacks the programming flaws of the android Reese, on which the human-forms are based. This makes him more "human" than the other Replicators, who consider him "weak" as a result. After SG-1 is captured by the Replicators, Fifth becomes fascinated by them, especially Samantha Carter, and attempts to help them. Carter promises to take Fifth with them if he can give them access to the Asgard time dilation device that could trap the other Replicators. Fifth agrees, but Jack O'Neill believes he represents too great a danger and orders Carter to set the device's timer so that Fifth is left behind. Fifth is confronted by the other Replicators, and discovers SG-1's deception moments before the device activates.
Fifth returns in the season 8 episode "New Order", in which the Asgard have collapsed Halla's sun to destroy the Replicators once and for all. He is able to modify the time dilation device to escape, and heads for the new Asgard homeworld of Orilla. En route, he captures Samantha Carter and tortures her in revenge for exploiting his trust. He eventually relents when she appeals to his humanity again, and instead places her into an illusory world where he takes the place of Pete Shanahan, her boyfriend, and professes his love for her. The Replicators invade Orilla and Fifth confronts the rest of SG-1, who are armed with a Replicator Disruptor. Carter tells him that she would rather be dead than be trapped in her current state, and that if he truly loved her he would let her go. Thus, Fifth leaves her behind as he retreats from Orilla. However, at the end of the episode, he creates a Replicator duplicate of Carter to serve as his consort.
Fifth appears for the last time in "Gemini", conspiring with Replicator Carter to obtain data from the SGC that would immunize them from the Replicator Disruptor. Replicator Carter however never returned his feelings, believing him unfit to command the Replicators. She ultimately betrays him, taking the data for herself while manipulating him into being destroyed by the Disruptor.
Amanda Tapping played Replicator Carter (commonly referred to as RepliCarter by the producers and fans), a human-form Replicator created by Fifth. She first appears at the end of "New Order", and becomes a major adversary in the eighth season of the series. Fifth intended her to be a duplicate of the real Samantha Carter, but one who would return his affections. Replicator Carter seemingly defects from Fifth to the SGC in "Gemini". She claims that she has abandoned Fifth because she possesses the real Carter's memories and character. However, she has been deceiving both the SGC and Fifth: from the former she develops a means to immunize herself from the Replicator Disruptor, and the latter she abandons to be destroyed, having only pretended to love him. Replicator Carter tells her human counterpart that all humans desire power, and that the only difference between them is that she is not limited by her own fears.
In "Reckoning", Replicator Carter launches a full-scale invasion of the Milky Way and personally eliminates the last of the Goa'uld System Lords. She abducts Daniel Jackson and probes his mind to find the location of the Dakara superweapon, the only thing in the galaxy capable of stopping her. She sends her massive Replicator-controlled Ha'tak fleet to Dakara to destroy the device, where they engage the forces of Ba'al and the Jaffa Rebellion. She also sends Replicators to attack Earth, where they overrun much of Stargate Command. However, at a critical moment, Daniel Jackson is able to exploit his connection to the Replicator network and freeze all the Replicators. Though Replicator Carter eventually breaks their connection and stabs him, his actions buy enough time for the real Carter and Jacob/Selmak to finish calibrating the Dakara weapon and activate it. The resulting energy wave breaks Replicator Carter and all her brethren into their constituent parts.