In 2026, a new machine called the Augma is developed to compete against the NerveGear and its successor, the Amusphere. A next-gen wearable device, the Augma doesn't have a full-dive function like its predecessors. Instead, it uses Augmented Reality (AR) to get players into the game. It is safe, user-friendly and lets users play while they are conscious, making it an instant hit on the market. The most popular game on the system is "Ordinal Scale" (aka: OS), an ARMMORPG developed exclusively for the Augma. Asuna and the gang have already been playing OS for a while, and soon Kirito decides to join them. However, they're about to find out that Ordinal Scale isn't all fun and games.

The film's story begins when Arata inadvertently touches "Hermes Apocrypha," Lilith's Grimoire. Suddenly, he is enveloped by a bright white light, and a girl appears before him. She calls herself Lilim, and treats both Arata and Lilith as her parents. At the same time she appears, something changes in the world. The forbidden Eternal Library awakens. In the Library is sealed the ultimate culmination of Alchemy, the White Demon Lord. The White Demon Lord plots to eliminate Arata and the Trinity Seven to usurp the position of Demon Lord. Bristling with untold power, the White Demon Lord attacks Arata, and triggers a desperate crisis where Arata and the Trinity Seven must save the world in this last battle.