Lonely Castle in the Mirror (かがみの孤城) is a Japanese novel written by Mizuki Tsujimura and published by Poplar Publishing in May 2017. A manga adaptation illustrated by Tomo Taketomi was serialized in Shueisha's manga magazine Ultra Jump from June 2019 to February 2022, with its chapters collected in five volumes. An anime film adaptation produced by A-1 Pictures was released on December 23, 2022.
Plot
Kokoro Anzai, a first-year at Yukishina Junior High School, stays at home due to bullying. She tries attending a child development support class, but has not returned to school since. One day, she finds the magical portal within the mirror at her room. While inspecting it, she is dragged into the mirror and finds herself in front of a castle on the island surrounded by water. There, she meets a girl with a wolf mask. The self-proclaimed Wolf Queen introduces her to six other children: Subaru Nagahisa, Akiko Inoue, Rion Mizumori, Earth Masamune, Fūka Hasegawa, and Haruka Ureshino. Addressing them as Little Red Riding Hoods, the Wolf Queen instructs them to find a key hidden in the castle, which will lead them to a room where one can make a wish, before the deadline of March 30 of the following year. If anyone makes a correct wish, it will be granted and all children will lose their memories of the castle. She also tells the children to leave before 5pm. Any child who stays at the castle on the evening will be eaten by a wolf.
The seven children start spending time together at the castle, occasionally looking for the key.