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12-year-old Mitsuki Koyama is a talented singer who dreams of becoming a pop idol, but she is afflicted with sarcoma, which is curable only through a surgery that could cost her vocal cords and her ability to sing. The tumor in her throat also affects her ability to breathe well and sing loudly. On top of that, her grandmother hates music, and is completely opposed to Mitsuki's wish to audition. Her dreams seem impossible to achieve, until one day she is visited by two shinigami whom only she can see. Without knowing Mitsuki can see and hear them, they discuss how she only has one year left to live.

Realizing she cannot wait any longer to fulfill her dream, she runs away from her house and the shinigami to try to audition for a singing competition. However, the shinigami catch her before she is able to audition. Mitsuki moves one of the shinigami, Takuto, to make a deal with her—if she goes quietly when her one year is up, he will help her become a singer so that she may leave the world without any regrets. He gives her the ability to transform into a healthy 16-year-old self who is not encumbered by a throat tumor. Though there is much competition, she is able to win over the judges with her enthusiasm for singing and her excellent voice. To conceal her true identity, she chooses the stage name Fullmoon.

Mitsuki decided to become a singer two years before the shinigami's arrival, when she was 10 years old. She made a promise with Eichi Sakurai, then a 16-year-old boy she met in her orphanage, that the next time they met, they would both be closer to their dreams. Eichi wanted to become an astronomer and Mitsuki wanted to become a singer. Shortly after, Eichi was adopted and emigrated to America before Mitsuki could express her feelings for him. Mitsuki hopes that by becoming a famous idol, Eichi will be able to hear her sing and realize her feelings for him.

Shortly after her first audition to become a singer, the plot of the manga and anime of this series separate. The two stories share common themes, but events develop differently as the series advances. In addition, several of the characters in the anime have different histories and personalities from their manga counterparts.

Source: Wikipedia