Jonathan "Jack" O'Neill is a clone of Colonel Jack O'Neill created by a rogue Asgard named Loki, to replace him while the real Jack O'Neill was being studied. Unbeknownst to Loki, the Asgard put safeguards in O'Neill's DNA to prevent genetic manipulation. This resulted in the clone only maturing to the level of a teenager while retaining all of the original's memories, so that Stargate Command initially assumed he was the original Jack, who had been reverted to youth for reasons unknown. During their investigation into what had happened, the young O'Neill recalled seeing an Asgard, prompting Dr. Daniel Jackson and Teal'c to investigate similar reports of alien abductions, to try and find an explanation for his transformation.