"Undead Unluck" centers on Fuuko Izumo, a young woman cursed with the ability "Unluck," which brings catastrophic misfortune to anyone who touches her. After years of isolation to protect others, Fuuko encounters Andy, an immortal man with the ability "Undead," making him impervious to death. Andy seeks to end his eternal life and believes Fuuko's Unluck can finally kill him. Their meeting sets off a chain of events that draws them into a larger conflict involving the Union, an organization tasked with negating supernatural threats known as "Negators."

The Union is composed of individuals with unique negation abilities, each countering a specific natural law or phenomenon. These Negators are pitted against UMA (Unidentified Mysterious Animals), entities that embody and enforce the rules of the world. The Union's ultimate goal is to confront God, who governs these rules and seeks to reset the world. As Fuuko and Andy join the Union, they uncover the complexities of their abilities and the true nature of their world.

Fuuko's Unluck evolves from a curse into a powerful tool, while Andy's immortality becomes both a burden and a weapon. Their dynamic shifts from a partnership of convenience to a deeper bond as they face increasingly dangerous missions. The series explores themes of fate, free will, and the consequences of defying natural laws. Supporting characters, such as Gina, Shen, and Void, add depth to the narrative, each grappling with their own abilities and motivations.

The plot intertwines action, mystery, and emotional stakes, with battles against UMAs and internal conflicts within the Union driving the story forward. As Fuuko and Andy grow stronger, they uncover secrets about the Union's origins, the nature of Negators, and the cyclical nature of their world. The series balances intense combat sequences with moments of introspection, highlighting the characters' struggles and growth. "Undead Unluck" presents a unique blend of supernatural elements and character-driven storytelling, creating a compelling narrative within a richly detailed world.