The cutting-edge Soul Translator uses a full-dive interface that connects with the user's Fluctlight, equivalent to their soul, and differs from the typical method of brain signal transmission. Rath, a private institute, seeks to perfect the device with the help of Kazuto Kirigaya, a Sword Art Online survivor who now works as a part-time employee to test the system's capabilities in the Soul Translator's fantastical Underworld realm. To maintain secrecy, all virtual world memories are deleted upon leaving. However, Kazuto has vague recollection of a name, Alice, which disturbs him in reality. During an evening walk with Asuna Yuuki, Kazuto faces a familiar foe and sustains life-threatening injuries. He later awakens in the Underworld with no apparent way out. Desperate to return to the physical world, Kazuto embarks on a quest to find a way back.