"Lonely Castle in the Mirror" centers on Kokoro Anzai, a seventh-grade girl who withdraws from school due to bullying and anxiety. One day, the mirror in her bedroom begins to glow, and she is transported to a mysterious castle. Inside, she meets six other teenagers, all facing similar struggles with school and personal challenges. A figure in a wolf mask, known as the Wolf Queen, explains the rules: they have one year to find a hidden "Wishing Key" within the castle. The one who finds it will have a single wish granted, but if they fail, their memories of the castle will be erased.

The castle serves as a sanctuary for the group, offering them a reprieve from their daily struggles. Each day, they explore the castle’s labyrinthine rooms, uncovering clues about the key’s location while forming bonds with one another. The narrative alternates between the castle’s fantastical setting and the real-world challenges each character faces, such as family conflicts, academic pressure, and social isolation. As the story progresses, the characters’ backstories are revealed, highlighting their vulnerabilities and the reasons they were drawn to the castle.

Kokoro emerges as a central figure, her empathy and determination helping to unite the group. The dynamics among the characters shift as trust is built and secrets are uncovered. The castle’s rules, however, introduce tension, as the Wolf Queen warns that breaking them will result in severe consequences. The group must navigate these constraints while grappling with their own fears and insecurities.

As the deadline approaches, the search for the Wishing Key intensifies, leading to moments of both collaboration and conflict. The story builds toward a poignant resolution, intertwining themes of friendship, resilience, and the search for belonging. The castle’s true purpose is gradually unveiled, tying the characters’ personal journeys to the overarching mystery of the Wishing Key and the Wolf Queen’s intentions. The narrative concludes with a blend of hope and melancholy, emphasizing the transformative impact of their shared experiences.