Ryōta Majima has always wanted a younger brother, so when his father remarries and he gains a new stepsibling named Akira Himeno, he is thrilled. Akira is introduced as a high school boy—beautiful but shy, having grown up in a difficult family environment. Eager to bond, Ryōta spends time with Akira, and the two quickly become close. Their relationship deepens, but Ryōta soon makes a startling discovery: during a bath, he realizes that Akira is actually a girl. Despite the shock, Ryōta’s desire to be close does not waver, and over time Akira develops romantic feelings for him. What began as a sibling-like bond shifts as Akira actively works to turn their relationship from brother-and-sister into that of lovers. The story follows the flirtatious, comedic, and sometimes heartfelt misunderstandings that arise from this secret, exploring how Ryōta and Akira navigate their changing feelings while dealing with their respective family histories—Ryōta carries trauma from his abusive mother, and Akira struggles with the abandonment by her father. Supporting characters, including Ryōta’s best friend Kousei and his little sister Hinata, add depth to the social circle as the pair moves from a mistaken sibling dynamic to a genuine romantic connection. The manga is a romantic comedy centered on the premise that the person you thought was your little brother is actually your stepsister, and she wants nothing more than to be your lover.