By June, 2026, Kirigaya Kazuto (Kirito) was offered a job at a private enterprise, named Rath, by Kikuoka Seijirou, the VR crimes division member who Kazuto had been cooperating with ever since the end of the Sword Art Online incident. The job was to test out a 4th generation experimental FullDive machine, called the Soul Translator (STL), which ran on completely different principles than the FullDive machines before it. His job was seemingly simple: remain in continuous FullDive during the weekend and his memories of the testing would be blocked as a security measure. However, unbeknownst to him, the company was not actually testing the machine itself, but creating a special Bottom-Up type Artificial Intelligence (AI) for military use. For that reason, the company had copied the soul of a newborn baby and made it as a template for creating new AI, which would then be nurtured in a specialised virtual world, called Underworld, where time flows a thousand times faster than in the real world due to Fluctlight Acceleration. As the AIs created by Rath were far too compliant with laws to be used for the military, Rath brought Kazuto into the world with his memories restricted to test why the residents of Underworld were unable to break any laws. He lived in the world for nearly eleven years of accelerated time as a child again, until his influence resulted in one of the residents, a girl named Alice, to break one of the rules of the Taboo Index.