Kusamori Sugara, who attends the prestigious all-girls school Hoshikawa Academy, lives with her "onee-sama" Shirazaki Yui due to the school's system known as the "Engage Sisters." Yui is the top star of the opera club, friendly and well-liked by everyone. While it's unclear why she chose Sugara as her "younger sister" (Ruby: Spica), the two have built a good relationship. However, things begin to change when Yui's twin sister, Shirazaki Hatsu, starts to approach Sugara. Hatsu, who usually pretends to be a cold and composed student council president, shows her warm side in front of Sugara! Sugara finds herself flustered by Hatsu's actions, and Yui also starts to change her attitude towards Sugara... Moreover, Hatsu and Yui begin to switch places to approach Sugara!?