"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 to develop artificial intelligence with human-like consciousness. Kirito, the protagonist, remains comatose in the real world after sustaining severe injuries in a previous confrontation. His consciousness is trapped in the Underworld, where he is unable to interact directly with events.

The Underworld is embroiled in a large-scale conflict between the Human Empire and the Dark Territory, a war orchestrated by external forces seeking to exploit the virtual world for military purposes. Asuna, Kirito's companion, enters the Underworld to assist him, joining forces with Alice Synthesis Thirty, a former Integrity Knight who has defected from the Axiom Church. Alice, now a central figure in the Human Empire, leads the defense against the Dark Territory's invasion, which is bolstered by powerful entities and dark forces.

The Dark Territory's army, led by the malevolent Administrator and her subordinates, seeks to conquer the Human Empire and seize control of the Underworld. Key antagonists include Vector, a manipulative and ruthless figure, and Gabriel Miller, a real-world antagonist whose actions in the virtual world are driven by personal motives. The war escalates as both sides deploy advanced tactics, powerful weapons, and strategic alliances, with the fate of the Underworld hanging in the balance.

Throughout the series, themes of loyalty, sacrifice, and the nature of consciousness are explored. Characters such as Eugeo, Kirito's close friend, and other Integrity Knights play significant roles in the conflict, highlighting the emotional and moral complexities of the war. The narrative also delves into the ethical implications of artificial intelligence and the blurred lines between virtual and real-world consequences.

As the battle intensifies, Kirito's recovery becomes a pivotal element, with his eventual return to action shifting the dynamics of the war. The series culminates in a climactic confrontation that determines the future of the Underworld and its inhabitants, resolving key plotlines while setting the stage for potential future developments. The story maintains a balance between high-stakes action and character-driven moments, providing a cohesive continuation of the "Alicization" arc.