In the military science fiction series Stargate SG-1, the Replicators are self-replicating machine that seek to convert all civilizations into more of themselves, thus posing a dire threat to all other beings. They were first mentioned indirectly in the season 3 episode , and first seen onscreen in . The Replicators are primarily the enemies of the Asgard race, but Earth must also contend with them on several occasions. The Asuran of Stargate Atlantis are also called Replicators, due to their similarities to the race on SG-1, but are not directly related.
The Replicators first appear in the season 3 finale , following hints by Thor in about an enemy worse than the Goa'uld in their home galaxy of Ida. SG-1 faces off against the Replicators who have infested Thor's mothership, the Beliskner, to prevent them from invading Earth. They destroy the deceleration drive on the Beliskner, causing it to burn up in Earth's atmosphere. In the following episode , a single surviving Replicator from the Beliskner infests a Russia submarine before being neutralized. At the same time, Samantha Carter helps Thor avert an imminent Replicator invasion of the Asgard homeworld, Halla. In the season 5 premiere , SG-1 encounters the Replicators after being thrown to another galaxy by a supernova. The Replicators take over their Goa'uld mothership and head for the Milky Way; to prevent them from reaching their destination, SG-1 sabotages the mothership's engines so that it crashes into the planet Delmak upon exiting hyperspace.
The episode reveals that the Replicators were created by the android Reese (played by Danielle Nicolet) 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. It is possible that shooting her was unnecessary though, as mentioned by Dr.Daniel Jackson after Reese's death in . 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 trying to 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 O'Neill (who had subsequently had the knowledge of the Ancients downloaded into his mind again) uses the knowledge of the Ancients to create the Replicator disruptor. 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.
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