Anna Sasaki, a quiet and introverted 12-year-old girl, is sent to live with relatives in a rural seaside town to improve her health and emotional well-being. Struggling with feelings of isolation and self-doubt, Anna spends her days sketching and wandering the marshes near her new home. One day, she discovers an abandoned mansion across the water, which seems to come alive at night. There, she meets Marnie, a mysterious and spirited blonde girl who quickly becomes her closest friend. The two form a deep bond, sharing secrets and spending idyllic days together, though Marnie’s presence is unpredictable and tied to the mansion’s enigmatic history.

As Anna grows closer to Marnie, she begins to uncover fragments of the mansion’s past and its connection to the local community. Strange occurrences and fragmented memories suggest that Marnie may not be who she appears to be. Anna’s investigation into Marnie’s identity leads her to piece together a story of love, loss, and family secrets spanning generations. The narrative gradually reveals the true nature of Marnie’s existence and her profound link to Anna’s own life, culminating in a poignant resolution that ties their fates together.

The story explores themes of loneliness, identity, and the enduring power of human connections. Anna’s journey is one of self-discovery, as she confronts her feelings of abandonment and learns to open herself to the people around her. Marnie, as a character, serves as both a catalyst for Anna’s emotional growth and a bridge to understanding her own family history. The interplay between the past and present is central to the narrative, with the mansion acting as a symbolic space where memories and emotions converge. The supporting cast, including Anna’s foster mother, her relatives, and the townspeople, provide additional layers to the story, enriching the exploration of relationships and belonging.