After accidentally finding an old NerveGear helmet that belonged to a SAO developer who died in the game, Kirito begins having dreams about the unexplored 25th floor of Aincrad. Together with Asuna and friends, he creates a private copy of the game to explore this mysterious level. They discover that this floor was an experimental playground for a unique AI named Echo, created to study human emotions and love. During their adventures, the couple relives key moments of their relationship...