Ever since he was a young child, Sora Aoi had always exchanged words of "I love you" with his attractive older sister Aki. Now, as he comes of age, he finds it much harder to exchange such comforts as they did in those simpler times. When Aki demonstrates her feelings toward him to go beyond a sister's love, Sora finds himself questioning his feelings for his sister. Realizing their affection for one another, they consummate their love in secret. In a society that would never understand, they must now keep their love a secret from their family and everyone they know. However, Sora soon finds himself exploring many forms of love with different kinds of women he met that is not accepted by society thus questioning his feelings for his sister.