In the year 2026, a new augmented reality device called the Augma has become a worldwide phenomenon. Unlike the full-dive virtual reality systems that came before, the Augma is a safe, user-friendly headset that overlays digital content onto the real world while users remain conscious. Its killer application is a new type of game called Ordinal Scale, an ARMMORPG where players use their real-world movements to battle monsters and compete for rank-based rewards. The game quickly becomes a popular part of everyday life, capturing the attention of Kirito, Asuna, and their friends from the Sword Art Online incident.

Kirito is initially reluctant to join the new game, but he is drawn in by troubling rumors: bosses from the original Sword Art Online death game have started appearing as special raid events within Ordinal Scale. For many players, these nostalgic battles are just a thrilling promotion, but Kirito becomes suspicious. The mystery deepens with the introduction of Yuna, a popular AI idol who serves as the mascot for Ordinal Scale. Her concerts and in-game support songs provide powerful buffs to players, making her a beloved figure in the community. As Kirito, Asuna, and their friends like Silica, Lisbeth, and Klein begin participating in these SAO boss raids, they encounter a formidable top-ranked player named Eiji, who fights with incredible skill and seems to know Kirito.

The carefree atmosphere of the game takes a dark turn when Asuna and other SAO survivors begin to mysteriously lose their memories of their time trapped in Aincrad. After a particularly intense battle where Asuna sacrifices herself to protect a friend, she wakes up unable to recall any of her experiences or her relationship with Kirito from the death game. Investigating the connection between the memory loss and the appearance of the SAO bosses, Kirito uncovers a sinister plot orchestrated by Professor Shigemura Tetsuhiro, a researcher with a painful past tied to the original Sword Art Online. Working with Eiji, the professor is using the Augma to scan and collect the brainwave data of SAO survivors, specifically the powerful emotional memories tied to the game's bosses, in an attempt to recreate the digital soul of his late daughter, who was the inspiration for the AI idol Yuna.

As the story builds to a climax at the New National Stadium during Yuna’s first live concert, the scheme escalates. The event becomes a trap where the most powerful SAO boss, the Floor 100 boss, is summoned into the real world through augmented reality. With Asuna's memories and the safety of all the survivors at stake, Kirito must race to defeat the boss in a desperate battle that blurs the line between the game and reality. He is aided by a change of heart from Eiji, who realizes the error of his revenge. In the final confrontation, the original spirit of the professor's daughter, preserved within Yuna, uses her song to empower the players, allowing Kirito to unleash a powerful sword skill and save everyone, restoring the lost memories in the process.