"Lonely Castle in the Mirror" centers on Kokoro Anzai, a seventh-grade girl who has stopped attending school due to bullying and anxiety. One day, she discovers that her bedroom mirror has transformed into a portal, leading her to a mysterious castle. Inside, she meets six other students of the same age, all of whom are also avoiding school for various reasons. The castle is overseen by a figure known as the Wolf Queen, who presents the group with a challenge: they must locate a hidden "Wishing Key" within the castle by a specific deadline. The one who finds the key will have a single wish granted, but there is a catch—if the key is not found in time, all their memories of the castle will be erased.

The castle serves as a sanctuary for the children, offering them a temporary escape from their struggles in the real world. Each character carries their own emotional burdens, ranging from family issues to feelings of inadequacy and isolation. As they explore the castle’s labyrinthine rooms and solve puzzles, they gradually open up to one another, forming bonds of friendship and trust. The narrative alternates between their time in the castle and their lives outside, revealing the complexities of their personal challenges and how their experiences in the castle begin to influence their real-world decisions.

The Wolf Queen’s rules are strict: the children can only stay in the castle until 5 p.m. each day, and they are forbidden from discussing their real names or personal lives. Despite these restrictions, the group grows closer, sharing their fears and hopes. However, tensions arise as the deadline for finding the Wishing Key approaches, and the pressure to succeed intensifies. The story explores themes of loneliness, resilience, and the transformative power of human connection, as the characters confront their inner struggles and learn to support one another.

The narrative builds toward a poignant climax, where the true purpose of the castle and the Wolf Queen’s intentions are revealed. The resolution ties together the emotional arcs of the characters, emphasizing the importance of empathy and understanding in overcoming adversity. "Lonely Castle in the Mirror" is a heartfelt exploration of adolescence, mental health, and the healing potential of shared experiences.