Aria of a Starless Night fundamentally re-contextualizes the start of the Aincrad arc by making Asuna the protagonist and introducing her friend, Mito. The core plot follows their initial days in SAO, where the skilled beta tester Mito trains the novice Asuna. The main twist and emotional crux of the film occurs when they are swarmed by monsters. Overwhelmed by fear at the thought of watching Asuna die, Mito abandons her. Asuna narrowly survives with 1 HP remaining, but is left traumatized by the betrayal.
This leads Asuna to become a cynical, solitary player, determined to die fighting rather than cower. It is in this state that she is found by Kirito, who pulls her back from the brink of despair. Their interactions largely follow the 'Aria of a Starless Night' story from the light novel. The film's climax is the raid on the first-floor boss, Illfang the Kobold Lord. During the fight, Asuna is about to be killed, but Mito makes a dramatic return, saving her and atoning for her earlier abandonment. After the victory, Kirito sees the rising animosity towards beta testers. To protect Mito and others, he deliberately takes all the blame, claiming the title of 'beater' and parting ways with a newly resolved Asuna. The final scene shows Asuna deciding not to reform her party with Mito, instead choosing to follow Kirito to the second floor, formally introducing herself and solidifying the beginning of their partnership.