"Arei no Kagami" is a supernatural mystery anime set in a world where mirrors serve as gateways to an alternate dimension. The narrative centers on a high school student named Kaito, who discovers an ancient mirror in his family’s attic. Upon touching it, he is drawn into a parallel world where reflections of people exist as independent entities. These reflections, known as "Mirrors," possess their own consciousness and memories, often harboring dark secrets or unresolved emotions tied to their human counterparts.

Kaito’s journey begins when he encounters his own Mirror, a version of himself that is more assertive and enigmatic. The Mirror reveals that the alternate dimension is collapsing due to a growing imbalance caused by unresolved human emotions. To prevent the destruction of both worlds, Kaito must navigate the mirror realm, confront the Mirrors of others, and help them reconcile with their human counterparts. Along the way, he is joined by a group of allies, including a classmate named Rina, whose Mirror harbors a tragic past, and a mysterious guide named Shiro, who claims to have once been human.

The plot unfolds through episodic encounters with various characters, each episode delving into the emotional conflicts and hidden truths of the individuals and their Mirrors. Themes of identity, duality, and the consequences of repressed emotions are explored as Kaito uncovers the origins of the mirror world and its connection to his own family history. The story builds toward a climactic confrontation with the entity responsible for the dimension’s instability, forcing Kaito to confront his own fears and insecurities.

The main characters are defined by their relationships with their Mirrors and their personal growth throughout the series. Kaito evolves from a passive observer to an active participant in resolving the conflicts between humans and their reflections. Rina’s arc focuses on her struggle to accept her past mistakes, while Shiro’s enigmatic nature gradually reveals his ties to the mirror world’s creation. The interplay between characters and their Mirrors drives the narrative, creating a layered exploration of human emotions and the consequences of self-deception.