"Sword Art Online: Extra Edition" serves as a recap and side story to the first season of "Sword Art Online," blending a summary of key events with new content. The narrative begins with Kirito, Asuna, and their friends preparing for a diving trip. During this time, the group reflects on their experiences in the virtual world of Aincrad, where they were trapped in a death game. Flashbacks revisit pivotal moments, such as Kirito and Asuna’s relationship development, their battles against powerful foes, and the eventual liberation from the game.

The new content centers on a mission within the underwater VRMMO game "ALfheim Online" (ALO). Kirito and his friends are tasked with retrieving a rare item, the "Aquamarine Key," from a deep-sea dungeon. Asuna, who is still adjusting to virtual reality after her traumatic experiences in Aincrad, joins the mission. The team faces challenges, including navigating underwater mechanics and battling a massive sea monster. The mission highlights the group’s teamwork and camaraderie, while also providing lighthearted moments and character interactions.

The special episode concludes with the group successfully completing their mission and enjoying their time together, reinforcing the bonds formed through their shared experiences in virtual worlds. The recap segments serve as a refresher for viewers, while the new underwater adventure offers a self-contained story that expands on the series’ world and character dynamics.