Kirito Kazugaya spends his days in simple, childish bliss, playing with his friends Eugeo and Alice and revelling in the quiet harmony of the land known as Underworld. This peaceful life is shattered when playful exploration sees the trio arrive on the cusp of entering the so-called Dark Territory. When a brief slip sees Alice fall within this territory, the sentence meted out to her is a harsh one: execution.
This, however, is not a memory he will easily recall, provided as it is by a new and incredibly immersive Full-Dive technology which he has been hired to test, and which leaves him with no tangible recollection of his adventures. Little does Kirito know that he will soon be returning to Underworld in unexpected and dire circumstances, reuniting him with his good friend Eugeo and setting him down the path of a battle not just to save the missing Alice, but also himself.