Kirito, along with his childhood friend, Eugeo, has been tasked with cutting down a colossal tree called the Gigas Cedar. They’re swinging their ax when their friend Alice arrives with lunch. As they eat, the three friends decide to go to a fabled cave.

Kirito and Eugeo realize Selka has taken off to the End Mountains alone, and decide to chase after her. As they enter the cave, they encounter goblins, presumably from the Dark Territory, and must engage in a battle.

Ronie and Tiese are late getting to the dorm. Kirito rushes out in search of them, but no sooner has he gone than Frenica appears. She tells Eugeo about how she's suffered at Humbert's hands, and that Tiese and Ronie went to confront him about it.

Kirito and Eugeo are now considered criminals for having violated the Taboo Index and are thrown into the underground prison in the Central Cathedral. Meanwhile, on the Ocean Turtle in the real world, Asuna is watching over Kazuto as he's being treated.

Though riddled with wounds, Kirito manages to activate his Perfect Weapon Control art. Continuing on their way to the top floor of the Central Cathedral, Kirito and Eugeo find themselves standing before a strange-looking floating platform.

When Kirito and Alice both unleash the Perfect Weapon Control art in the Cloudtop Garden, the impact rips a hole into the seemingly impregnable Central Cathedral wall, which sends them both flying outside.

Eugeo reaches the ninetieth floor of the Central Cathedral, which turns out to be a vast bathing area. The perplexed Eugeo encounters Bercouli Synthesis One soaking in the bath. Meanwhile, Kirito and Alice are still climbing the Central Cathedral wall.

Seeing Alice’s reaction when he mentioned the name Selka, Kirito decides to tell her the truth about the Integrity Knights. Meanwhile, after being knocked unconscious during his battle with Bercouli, Eugeo awakes on the top floor of the cathedral.

Kirito and Alice reach the communal bath on the nintieth floor. There, they find the Integrity Knight Commander, Bercouli, who's been turned to stone by Chudelkin. Alice breaks down in tears upon seeing her mentor's tragic plight, but then...

Standing in Kirito and Alice's way is Eugeo, who's undergone the Synthesis Ritual and become an Integrity Knight. Kirito is rattled, until Alice admonishes him into resolving to fight Eugeo with everything he's got.

On the top floor of the Central Cathedral, Kirito, Eugeo, and Alice finally face Administrator. But Chudelkin uses the sacred arts to transform himself into a giant clown and attacks.

Cardinal appears on the top floor of Central Cathedral for a showdown with her bitter foe of the last two hundred years. She moves to achieve her objective of deleting Administrator, only to see the fallen sword golem rise up again.

Carrying the hopes of Cardinal, Eugeo, and Alice in his heart, Kirito stands yet again and visualizes the Black Swordsman, the top player who cleared SAO. For the sake of everyone he’s encountered in this world, Kirito, faces off with Administrator.

Recap of Sword Art Online: Alicization, aired one week before Sword Art Online: Alicization - War of Underworld.

Six months have passed since the fierce battle against Administrator. Alice has brought Kirito to her childhood home, Rulid Village, where they live peacefully. Gazing out at the world that she saved with Kirito, Alice reflects on all that’s happened.

A force from the Dark Territory is attacking Rulid Village. Though she’d lost her purpose in battle, in order to fight for her family, and for the people that Kirito and Eugeo tried to protect, Alice takes up the Osmanthus Blade once again!

Shasta, commander of the Dark Territory's dark knights, had intended to use Administrator's death as a chance for peace with the Human Empire. But his plans are thwarted by Emperor Vecta's return to the land of darkness.

Resolved to fight for humankind, Alice joins the forces of the Human Empire, bringing Kirito with her. But while the forces of darkness boast overwhelming numbers, only thirteen Integrity Knights are available to fight for the Human Empire.

The Final Load Test has begun, and the Eastern Gate has collapsed. Incited by Gabriel, now Emperor Vecta, the massive army of the land of darkness pushes on towards the Human Empire.

Integrity Knight Renly Synthesis Twenty-Seven, in battle for the first time, flees his position, allowing goblins from the forces of darkness to infiltrate to the tent where Kirito is.

The powerful formal art unleashed by the Integrity Knight Alice inflicts severe casualties on the forces of darkness. Alice encounters an enemy survivor and learns that Emperor Vecta's objective is to find the "Priestess of Light."

Upon seeing Alice, Gabriel orders his entire army to capture her. The pugilists guild races ahead of the main force. As Alice and Bercouli try to come up with a strategy to counter them, the silent Integrity Knight Sheyta Synthesis Twelve steps forward.

Asuna has logged into the Underworld using the superuser account of Stacia, the Goddess of Creation. After she descends, she is reunited with Kirito, thanks to Ronie and Tiese. But soon Alice arrives, leading to a precarious situation!

Asuna has created a bottomless chasm. The dark knights and the pugilists attempt to cross over to the other side, where the Human Empire Army waits. Meanwhile, Critter releases an announcement about a limited-time beta test for a bizarre new VRMMO.

Due to Gabriel's scheme, U.S. players were given black knight accounts, and are now continuously logging in to the Underworld. They begin mercilessly slaughtering soldiers from both the Human Empire Army and the Dark Territory Army.

Despite its modest size, the Human Empire Army and Asuna valiantly battle the wave of American players surging towards them. But they’re hopelessly outnumbered, and the Human Empire soldiers start to fall one after another.

Bercouli pursues Vecta, who’s abducted Alice. After a long, vicious battle, Bercouli unleashes his secret sword skill, Uragiri, and finally defeats Vecta, despite having sustained severe injuries himself.

Escaping his nightmares, Kirito regains consciousness at last. With the Night-Sky Blade in his right hand and the broken Blue Rose Sword in his left, he faces off against Vassago, armed with the Mate-Chopper, now enormous in size.

Alice and Asuna head to the World’s End Altar as Kirito confronts Gabriel. Unless he can defeat Gabriel and return to the real world before the maximum-acceleration phase begins, Kirito will have to spend the next 200 years in the Underworld.

The “Rath Attack Incident,” masterminded by the American National Security Agency, is over. Now head of the newly formed Oceanic Resource Exploration & Research Institution, Rinko unveils Alice to the world as an authentic artificial general intelligence.

Rinko informs Kazuto that Alice is missing. He’s about to rush to the Rath Roppongi branch when the doorbell rings. In a panic, Kazuto throws open the door, only to find a deliveryman there with a huge cardboard box.