The story takes place in the Taishou era (1912–1926). As soon as Fumiko Hayama enters an all-girls school, she falls in love with her beautiful upperclassman, Tsuyako Ichinose, in the schoolyard. Fumiko then learns about the custom of sister-like relationships between female students called "S", and she searches the school to get Tsuyako to become her "sister". However, since Tsuyako's family is wealthy, she believes that anyone who offers her an "S" relationship is just after her fortune...

(Source: Natalie.mu, machine translated and edited)