"Sakura no Chikai" centers on a young woman named Haruka Sakura, who lives in a small rural town surrounded by cherry blossom trees. The town is steeped in a centuries-old tradition where every spring, a ceremonial vow is made to the spirits of the blossoms to ensure prosperity and peace. Haruka, a reserved and introspective individual, is unexpectedly chosen as the next guardian of this tradition, a role passed down through her family for generations. Her selection comes with the responsibility of maintaining the balance between the human world and the spiritual realm, which is threatened by an ancient curse tied to the town's history.

The narrative unfolds as Haruka begins to uncover the truth behind the curse, which is linked to a tragic love story from the past involving her ancestor and a spirit. As she delves deeper, she encounters Kaito Tachibana, a mysterious outsider who arrives in town with his own hidden agenda. Kaito possesses knowledge of the curse and offers to assist Haruka, though his true intentions remain ambiguous. Their uneasy alliance gradually evolves into a partnership as they work together to unravel the mysteries of the curse and prevent its resurgence.

Throughout the series, Haruka and Kaito face numerous challenges, including confrontations with malevolent spirits and the skepticism of townsfolk who doubt the validity of the ancient traditions. Haruka's journey is one of self-discovery, as she learns to embrace her role and the strength within herself. Kaito, on the other hand, grapples with his own past and the guilt he carries, which is intricately tied to the curse. The story explores themes of duty, sacrifice, and the enduring power of human connections, set against the backdrop of the town's ethereal cherry blossom landscapes.

The supporting cast includes Haruka's childhood friend, Yuki, who provides emotional support and a grounded perspective, and an elderly shrine maiden, Ayame, who serves as a mentor figure, guiding Haruka through the complexities of her role. The interplay between the characters and their shared history with the town creates a layered narrative that intertwines personal growth with the resolution of the overarching conflict. The series culminates in a climactic confrontation where Haruka must make a pivotal choice to either uphold the tradition at great personal cost or find an alternative path to break the cycle of the curse.