Before Mitsuki Kouyama could confess her feelings to Eichi Sakurai, her friend had to move to America two years ago. Distance didn't stop their dream pact, though - Mitsuki wanted to be a professional singer while Eichi aimed to be an astronomer. She yearned for the day when her music would radiate to him like a full moon, despite their separation. But Mitsuki's journey to achieving her dream hit a hurdle; she suffered from throat cancer, weakening her voice and making singing a struggle. Even though her grandmother (who despised music) insisted that she undergo surgery to remove the cancer, Mitsuki turned it down, fearing that she may lose her voice forever. Suddenly, two shinigami, Meroko Yui and Takuto Kira, appeared, revealing that Mitsuki had only one year left to live. This chilling revelation triggered a new drive within her, which forced her to push herself harder with the help of Meroko and Takuto, to become a professional singer before time ran out. Full Moon wo Sagashite portrays Mitsuki's heart-wrenching story and her journey with her shinigami-companions as they uncover the significance of singing and the value of seizing opportunities in their existence.