Keita Ibuki is an independent and struggling freelance computer programmer who had a chance meeting on one night with a high Mototsumitama named Kuro. He gives her his dinner, a bowl of ramen, and tries to protect her when she is attacked in an ambush by an unknown Mototsumitama. Keita’s heart is severely damaged during the fight between the Mototsumitamas, but Kuro was able to save his life by exchanging his heart with her’s since she has enhanced healing abilities like all Mototsumitamas. This creates a contract between them, which makes her more powerful than before. However, Kuro warns Keita that they need to be together all the time in order to be at their strongest.