"Rei Zero" centers on Subaru Natsuki, a young man who is suddenly transported to a fantasy world after leaving a convenience store. Upon arrival, he encounters a half-elf girl named Emilia, who is a candidate to become the ruler of the Kingdom of Lugnica. Subaru quickly forms a bond with her and vows to help her achieve her goal. However, he soon discovers a mysterious ability: whenever he dies, he is sent back in time to a specific point, retaining memories of previous events. This power, referred to as "Return by Death," becomes both a curse and a tool as Subaru navigates the dangers of this new world.

The narrative unfolds through a series of arcs, each presenting escalating challenges. Subaru's repeated deaths force him to confront his own limitations and the moral complexities of his actions. His relationships with key characters, including Emilia, the maid twins Rem and Ram, and the enigmatic Beatrice, deepen as he strives to protect those he cares about. The story explores themes of perseverance, sacrifice, and the consequences of one's choices, as Subaru repeatedly faces life-threatening situations and emotional turmoil.

The world-building is intricate, with political intrigue, magical elements, and a diverse cast of characters, each with their own motivations and backstories. The Witch Cult, a malevolent organization, serves as a recurring antagonist, while the dynamics between the royal candidates and their supporters add layers of complexity to the plot. Subaru's journey is marked by both triumphs and failures, as he learns to harness his ability to alter outcomes while grappling with the psychological toll of his experiences.

"Rei Zero" balances intense action with emotional depth, creating a narrative that is both gripping and thought-provoking. The series is known for its unpredictable twists and the gradual unraveling of its mysteries, keeping viewers engaged as Subaru's story unfolds.