Lakeside Park City is renowned for its unparalleled levels of happiness under the reign of Queen Siren. The Queen's persuasive words encourage residents to prioritize their happiness above everything else, but those who resist risk facing severe consequences, including execution through beheading, strangulation, or firing squad.
Queen Siren depends on a large network of Ondine terminals placed strategically throughout the city to maintain order and prevent any act of rebellion. Hatsune Midorikawa runs one of these terminals while disguised as a student, assigned to identify dissidents. However, her task is complicated when she falls for Len Kinami, whom she met as a child. Queen Siren's neat world becomes chaotic when she discovers Hatsune's emotions.
