"Full Moon wo Sagashite" centers on Mitsuki Koyama, a 12-year-old girl with a terminal throat tumor who dreams of becoming a professional singer. Her condition prevents her from pursuing her passion, as singing could worsen her health. Mitsuki’s life takes an unexpected turn when she encounters two shinigami, Takuto and Meroko, who reveal that she has only one year left to live. The shinigami are tasked with guiding souls to the afterlife, but they grant Mitsuki a temporary reprieve by transforming her into a healthy 16-year-old version of herself, allowing her to chase her dreams under the alias "Full Moon."

As Mitsuki begins her journey as a singer, she is driven by her love for Eichi Sakurai, a boy she met in her childhood who moved to America and inspired her to pursue music. Her transformation into Full Moon not only enables her to perform but also brings her closer to her goal of reconnecting with Eichi. However, her time is limited, and the shinigami remain by her side, grappling with their own unresolved pasts. Takuto, in particular, struggles with memories of his human life, which are gradually revealed to be intertwined with Mitsuki’s fate.

The series explores themes of love, loss, and perseverance as Mitsuki navigates the challenges of the music industry while confronting her mortality. Her relationships with Takuto and Meroko deepen, revealing their own emotional struggles and connections to her life. The narrative balances Mitsuki’s professional aspirations with her personal journey, culminating in a poignant exploration of life’s fleeting nature and the power of dreams. The story’s emotional depth is heightened by its exploration of the shinigami’s roles and their evolving understanding of human emotions, creating a layered and compelling narrative.