In the state of Lakeside Park City, local residents experience unprecedented rates of happiness under the rulership of Queen Siren. Her words are like a mind-numbing sense of aromatherapy, beckoning the populace to fulfill one duty: to be happy. Those who do not comply can choose their form of execution; strangulation, beheading, firing squad—the options are endless!

To prevent the spread of unhappiness, Queen Siren keeps constant watch using numerous Ondine, information terminals that act as her eyes and ears across the city. One such Ondine goes by the name Hatsune Midorikawa, who operates undercover at a school to weed out dissidents. However, over time, a certain fuzzy feeling begins to sprout within Hatsune whenever she looks at a boy named Len Kinami, whom she met once as a child. When these feelings are transmitted to Queen Siren, her mind spirals into turmoil...

[Written by MAL Rewrite]