"Sword Art Online: Alicization - War of Underworld" is the second part of the "Alicization" arc, continuing the story from "Sword Art Online: Alicization." The narrative unfolds in the virtual world known as the Underworld, a highly advanced simulation created using the Soul Translator technology. Kirito, the protagonist, remains comatose after a confrontation with Administrator Quinella, leaving his allies to fight in his stead. Asuna, along with other familiar characters from the real world, logs into the Underworld to support Kirito and protect the virtual realm from external threats.

The central conflict revolves around the invasion of the Underworld by external forces seeking to exploit its advanced AI and data for military purposes. The Dark Territory, a hostile region within the Underworld, launches a full-scale war against the Human Empire, where Kirito and his allies reside. The Human Empire, led by Integrity Knight Alice Synthesis Thirty, prepares to defend their land against the overwhelming forces of the Dark Territory. Alice, a former enemy turned ally, plays a pivotal role in organizing the defense and rallying the Human Empire's forces.

As the war intensifies, the narrative shifts between the Underworld and the real world, where the Rath organization struggles to protect the Underworld from external interference. The stakes are heightened as the real-world antagonists, including the mysterious Gabriel Miller, seek to manipulate the Underworld for their own gain. Gabriel, driven by personal motives, becomes a significant threat, both in the virtual and real worlds.

The series explores themes of identity, morality, and the consequences of blurring the lines between reality and virtual existence. Characters such as Eugeo, though no longer physically present, continue to influence the story through their impact on Kirito and Alice. The emotional weight of their past actions and relationships adds depth to the ongoing conflict. Meanwhile, Asuna's determination to save Kirito and protect the Underworld highlights her growth as a character and her unwavering loyalty.

The war reaches its climax as Kirito gradually regains consciousness, reawakening to join the battle. His return marks a turning point in the conflict, as his unparalleled skills and resolve inspire his allies and shift the momentum in their favor. The series concludes with a resolution to the immediate threats, though lingering questions about the future of the Underworld and its inhabitants remain. The narrative sets the stage for potential future developments, leaving room for further exploration of the virtual world and its connection to reality.