Amane Nakanose's mother wants to remarry and, elated at the news that she will gain a younger brother, Amane happily approves. But when Naoya Akasegawa introduces himself as her brother, he is far from the image of the younger brother that she dreamed of, being only three days younger than her. Similarly, the older sister that Naoya had hoped for turns out to be very different than he had imagined.

Faced only with cold neglect and a demand to be referred to as "onee-chan," Naoya tries to get used to daily life with his new sister. But for all of Amane's haughty demeanor, he cannot help but notice the sweet girl within. With reawakened hope in his heart, Naoya decides to make the best of his new family.