Twelve-year-old Mitsuki Koyama dreams of becoming a singer to reunite with her first love, Eichi Sakurai. However, she is afflicted with sarcoma, which affects her ability to breathe well and sing loudly. Her tumor is curable only through a surgery that could damage her vocal cords. At the same time, her grandmother hates music and is completely opposed to Mitsuki’s wish to audition. Mitsuki’s dreams seem impossible to achieve, until one day she is visited by two shinigami, Takuto and Meroko, whom only she could see. Takuto and Meroko, inadvertently tip Mitsuki off that she has only one year left to live.
Mitsuki then realizes she cannot wait any longer to fulfill her dream, so she runs away from home and the shinigami, to try to audition for a singing competition. She moves Takuto to agree to a compromise: if Mitsuki promises to go quietly when her year is up, he would help her become a singer, so she could leave the world with no regrets. Takuto gives her the ability to transform into a completely healthy 16-year-old, so that she could meet the age requirements of the audition and sing without hindrance. Mitsuki wins over the judges with her excellent voice and her enthusiasm for singing, sealing a contract with Seed Records. To conceal her true identity, she chooses the stage name “Fullmoon.”