It begins when Tsukiko enters the nurses office and finds Kotarou Hoshizuki, the school doctor, sleeping lazily on one of the patient beds. Kotarou then takes over and reminisces about a past memory that both has brought him great joy but also terrible sadness: his first meeting with the twins Iku and Yui Mizushima. Before meeting Iku and Yui, Kotarou had been living a monotone life and didn't really look towards his future. It was at that point in time that his older sister introduced him to Iku and Yui and he began to develop a very close relationship with the two. Even so, Yui had a terrible heart condition and Kotarou knew her life was limited. Having become so close to the two he set his heart to do something for them thus his wanting to study and become a doctor to save Yui's life. (thetvdb.com)