"Watashi ga Ningyo ni Natta Hi" centers on a young girl named Wakasa, who lives in a small coastal town. One day, she discovers that she has transformed into a mermaid, a change that brings both wonder and challenges. The story explores her struggle to adapt to this new reality while keeping her condition a secret from those around her. Wakasa’s transformation is tied to a mysterious legend about mermaids in her town, and as she investigates, she uncovers deeper connections to her family and the sea.

The narrative delves into themes of identity, belonging, and the tension between human and supernatural worlds. Wakasa’s relationships with her family, particularly her grandmother, play a significant role, as her grandmother holds knowledge about the mermaid legend and its implications. As Wakasa navigates her dual existence, she encounters other characters who are either aware of her secret or become entangled in her journey. These interactions reveal layers of the town’s history and the sacrifices tied to the mermaid myth.

The plot balances Wakasa’s personal struggles with broader mysteries, including the origins of the mermaid curse and its impact on the town’s inhabitants. The story progresses through a mix of emotional introspection and moments of discovery, as Wakasa learns to reconcile her human emotions with her newfound mermaid nature. The series maintains a focus on her growth and the evolving dynamics of her relationships, set against the backdrop of a town steeped in folklore and secrets.