Replicators were the enemy of the Asgard, far more dangerous than the Goa'uld. They were first created by Reese, a sophisticated but childlike android who constructed them as toys using nano-technology to reorder matter on a microscopic level. She could control them with her mind, but as she taught her toys to replicate and to protect her, she began to lose control. The Replicators destroyed everyone on her homeworld, then spread through the stargate to other planets. They were discovered on an isolated planet in the Asgard home galaxy some time ago and were brought aboard an Asgard ship for study before the danger was fully comprehended.

Replicators appeared as mechanical spiders and had an extremely high capacity for learning, with the ability to incorporate or modify the technology they consumed. They were a kiron-based technology, a kiron being an energy particle unknown to Earth technology. Each individual building block was capable of exerting a reactive modulating mono-polar energy field on other blocks, allowing them to assemble themselves into many forms, in order to perform various tasks, both individually and as a group, ultimately for the single purpose of self replication. They ingested metals and alloys, creating more of themselves and taking on the characteristics of the materials they consumed. As they reached a critical number, they continually sought more advanced technology and raw materials, and they were capable of modifying technology to meet their needs. In an attack, they would produce a powerful spray, although they usually would not attack unless threatened. They were unaffected by advanced energy weapons, and although the more primitive Earth projectile weapons were effective in shattering them into smaller pieces, those pieces were capable of reassembling themselves.

SG-1 defeated the Replicators in a number of encounters, by luring them to the destruction of the Asgard's most advanced ship, the O'Neill, by destroying an infested Russian submarine with explosives, by destroying the Biliskner and the pyramid ship carrying Apophis in uncontrolled reentries, and by destroying Reese to disable her control over them on Earth. However, the Replicators continued to be a plague in the Asgard galaxy, annihilating everything in their path.

When the Asgard homeworld was overrun by Replicators, the Asgard High Council devised a plan. Based on information learned from studying the remains of the android Reese, a trap was set and the Replicators were called forth to the planet Hala where they would be trapped in a time dilation bubble. However, the Replicators reversed the time dilation device and used Reese's technology to evolve into human form. Six human-form Replicators were created, composed of millions of microscopic cell units similar to nanites. These Replicators presented an even greater threat. They were resistant to the projectile weapons that had previously been effective against them, they were capable of passing through solid matter, they were connected by shared thought transmitted over vast distances through a subspace link, and they were capable of reaching inside the human mind to retrieve knowledge. The Asgard turned to SG-1 to infiltrate the Replicators and repair the time dilation device, and by betraying Fifth, the only human-form Replicator to exhibit the quality of humanity, SG-1 succeeded in reactivating the device and trapping the Replicators in a time dilation field on Hala.

Because the time dilation field was only a temporary measure, expected to last approximately two years, the Asgard devised the more permanent solution of collapsing Hala's sun and drawing the entire planet into a black hole. However, before reaching the event horizon of the black hole, a large number of Replicators were able to form a coherent mass and to use the time dilation device to counteract the intense gravitational effects, allowing them to escape. The blocks formed a ship under the control of Fifth, who sought revenge for his betrayal. Fifth captured Carter and journeyed to the Asgard planet of Orilla, a world rich in the neutronium required to create new human-form Replicators. Although Carter was rescued and the new disruptor technology successfully eliminated the Replicators from Orilla, Fifth escaped and created his own human-form Replicator in the image of Carter.

As Fifth arrived in the Milky Way galaxy, bent on revenge, he sent Replicator Carter in advance for the purpose of rendering Earth's disruptor technology ineffective. However, Replicator Carter deceived both SG-1 and Fifth when she enlisted Carter's cooperation, isolated the cipher that would make her immune to the disruptor, and used the device to destroy Fifth before escaping through the stargate. Her true intention had been galactic domination, and in her quest for superior technology, she and her brethren launched an attack against the Goa'uld.

As the numbers of Replicators expanded exponentially and they began systematically eliminating the System Lords and assimilating their technology at an alarming rate, Replicator Carter sent Replicators through the stargate to Earth and launched a full scale attack against the forces of Ba'al and the Rebel Jaffa at Dakara, the location of the only device in the galaxy capable of stopping her. Seeking new technology, she captured Daniel, believing that his subconscious mind held the knowledge of the Ancients. However, as Replicator Carter entered Daniel's mind, he learned how to enter hers, and through her, he seized control of every Replicator in the galaxy and held them in suspension until Replicator Carter killed him to release his grip. In those few moments, Carter and Jacob were able to modify and activate the Ancient device on Dakara, and the energy wave translated throughout the galaxy, instantly disintegrating every Replicator into individual harmless blocks. The enemy that had plagued the galaxies for generations was at last eliminated.

Cross Reference: Aegir, Apophis, Asgard, Ba'al, Biliskner, Black Hole, Jacob Carter, Dakara, Dakara Weapon, Paul Davis, DEFCON, Disruptor, Eighth, Fifth, First, Hala, Ida, The Daniel Jackson, Nano-Technology, Neutronium, The O'Neill, Orilla, Penegal, Prometheus, Rebel Jaffa, Reese, Replicator Carter, System Lords, Thor, Time Dilation Device, Tok'ra

Episode Reference: Nemesis, Small Victories, Enemies, Menace, Unnatural Selection, New Order, Gemini, Reckoning