Haruki unexpectedly becomes step-brothers with Hikaru, a boy ten years younger than him, when his father gets remarried in his fifties. Although Hikaru is typically reserved, he quickly takes a liking to Haruki. He acts like an exemplary younger brother, inviting Haruki to engage in activities and hang out with him. However, Hikaru's feelings for Haruki may go beyond those of a sibling, and his intentions may not be purely virtuous.