Board Thread:Questions and Answers/@comment-25072311-20141221071302/@comment-6697950-20141221072206

While Kazuto was walking Asuna home they were attacked by Kanamoto Atsushi (Johnny Black), the last member of Death Gun who still had a vial of Succinycholine left. During the fight, Kazuto was injected with the drug and his heart temporarily stopped. Although the doctors managed to restart it, the several minutes he spent without blood circulation caused brain damage. But then, Kazuto was secretly kidnapped by the JSDF, since they were working on a FullDive machine that interacted with the soul (called a Fluctlight) of the user and thus had the ability to stimulate Kazuto's recovery. However, since he was connected to the machine, his consciousness woke up in a virtual world created by the JSDF to nurture highly intelligent AI.

And... The story is awfully long, so if you want more spoilers, you could just read the novel summaries on the wiki, since I don't see the point in rewriting them...