Hayato Hashiro is an honor student. His family is a prestigious big conglomerate, his grades are reasonably good, and he is good at sports. But no matter what he does, he can never be the best. He has a childhood friend who is his fiancee. She is a kind, perfect, and beautiful genius girl. No matter what she does, she is the best. However, Hayato has a strong sense of inferiority to his fiancee and feels suffocated. Then, an older half-Japanese girl named Chiharu Maria moves into Hayato’s house as his stepsister. She is a beautiful girl, but she is also devilish and naive. The free-spirited Maria aggressively approaches the younger Hayato, taking baths with him and sleeping in the same bed with him. She spoils Hayato to no end and begins to dote on him. In fact, Maria confesses that she had met Hayato when she was a child and that she had liked him since then. Hayato is also attracted to Maria, who acknowledges him for who he is. The two become more than siblings and less than lovers. However, it was revealed that they are half-siblings by blood. At the same time, Hayato’s fiancee and younger stepsister begin to fight back to get Hayato back.