Ten days before graduation, homeroom teacher Ibuki Hiiragi takes his 29 students of Class 3-A hostage. He seals off the classroom and declares that the students are now his hostages, forcing them to confront the truth behind the mysterious suicide of a popular classmate, Reina Kageyama, which occurred months earlier. As the clock ticks down, Hiiragi forces the students to engage in a series of 'lessons' that expose the toxic culture of bullying, social media harassment, and apathy within their school. Through psychological warfare and intense confrontation, he aims to teach them the weight of their words and actions before they enter the adult world, while the police and the public struggle to understand his true motives.