Hayafuji Kasane is an office lady who hides her true nature behind a cool and aloof exterior. In reality, she is a depressed and bumbling closet otaku who hates her soul-crushing desk job. Her only secret joy is visiting a family restaurant to see Aoyama Meguru, a high school girl who works there as a waitress. One rainy night, Kasane finds Meguru standing alone outside the restaurant and offers her umbrella. Instead of accepting it, Meguru makes a startling request, asking Kasane if she will become her big sister.

This unusual proposal leads the two to sign a contract and begin living together as contract sisters. The story follows their new domestic life as Kasane moves into the apartment where Meguru lives alone, with a relative serving as the landlord. On the surface, Kasane plays the role of a cool big sister, but her feelings toward Meguru are secretly not purely sisterly. For Meguru, Kasane reminds her of her real elder sister, which seems to be a significant factor in her desire to form this bond.

As the series progresses, the narrative explores the gap between Kasanes public persona and her private identity as an awkward otaku, often leading to comedic situations. The manga also introduces other characters such as Miichan, a friend from Kasanes high school days who has since adopted a masculine-presenting style. The story continues to develop the relationship between the two main characters as they navigate their unusual arrangement, with Kasanes hidden romantic feelings creating underlying tension beneath their sisterly contract. The manga began serialization in 2022 and remains ongoing, with multiple collected volumes released.