Monster opens with a building on fire, as fifth grader Minato (Soya Kurokawa) and his single mom, Saori Mugino (Sakura Andō, also known for Shoplifters and Godzilla Minus One), watch from their balcony. After the fire, rumors that Minato’s teacher Mr. Hori (Eita Nagayama) frequented the disreputable hostess bar in the burning building start spreading. Saori has heard the rumor and this may have planted a seen of distrust for Mr. Hori in Saori’s mind. One night, Saori comes home and finds hair clippings in the bathroom and realizes that Minato isn’t home. She goes out looking for him and finds him in an abandoned tunnel with a bandaged and…