Anna, a creative 12-year-old misfit feeling isolated from everyone around her, leaves Sapporo to spend summer break with her mother's relatives in the rural seaside town of Kissakibetsu. There, the ailing, socially awkward schoolgirl stumbles upon the deserted Marsh House across the shore, where Anna spots Marnie, an enigmatic blonde girl longing for companionship and friendship. Little by little, a mysteriously profound bond between the two young ladies forms, blurring the delicate boundaries of reality and fantasy. But Anna must know. Who is the Girl in the Blue Window, the old mansion's cryptic, eerily charming inhabitant?