"Kamisama no Inai Nichiyōbi" is set in a world where God has abandoned humanity, leaving the dead unable to pass on and the living unable to die naturally. In this world, gravekeepers are tasked with burying the dead to ensure their souls can rest. The story centers on Ai Astin, a 12-year-old girl who discovers she is a gravekeeper after her village is destroyed. Accompanied by a mysterious man named Hampnie Hambart, who claims to have killed her mother, Ai embarks on a journey to understand her role and the world's strange phenomena.

The narrative explores the consequences of a world without divine intervention, where the dead rise as immortal beings unless properly buried by a gravekeeper. Ai's journey brings her into contact with various characters, including Yuri, a boy who becomes her companion, and Scar, a gravekeeper with a tragic past. Together, they navigate a world filled with undead, moral ambiguity, and the lingering question of why God left.

Themes of life, death, and the meaning of existence are central to the story. Ai's growth as a gravekeeper and her interactions with others reveal the complexities of a world where the boundaries between life and death are blurred. The series delves into the emotional and ethical struggles of its characters, particularly as they confront the consequences of immortality and the absence of a higher power. The plot is driven by Ai's quest to uncover the truth about her mother, Hampnie's past, and the ultimate fate of humanity in a godless world.