The protagonist, Kuronagi Mikoto, is such a hardcore brocon that her friends often make fun of her for it. In particular, she has special feelings for her oldest brother, Soushi.
On Christmas Eve, Mikoto happens to see Soushi walking along with a lady whom appeared to be his lover and gets quite the shock. Soushi's lover, Tatsumi Rinko, studies in the same university laboratory as him.
Since Rinko has an "adult" air about her that Mikoto doesn't, Mikoto becomes restless and jealous of her. She then realises that the feelings she have for Soushi isn't simple brotherly love. Even though she knows that he's her biological older brother, her taboo feelings keep on growing...
Also, Mikoto's other older brother, Wakazumi's feelings for her are also growing stronger and...?
Where will the three's entangled feelings lead them?
[Translation of the official website's story summary.]