Due to possessing the diamond of the heart, Himeno experiences chest pain when touched by boys. Despite this, she formed a bond in her youth with Mamoru, a kind soul, and Keigo, who is straightforward, both capable of touching her without adverse effects. Oblivious to the underlying cause, they pledged to safeguard Himeno. As a threat arises, putting Himeno's heart in peril, will anyone defend her besides her chivalrous knights?