Young Anna, who resides in Sapporo, is experiencing issues with her peers. A doctor advises Anna to relocate to a more tranquil area since she experiences panic episodes. She spends her time going around the neighborhood and doing sketches after relocating to the country with her family and being separated from her mother. She is drawn to a mansion across the lake that appears to be deserted and can only be reached on foot during low tide. There, she first encounters Marnie. Marnie, who even asks Anna to keep her a secret from the community, strikes her as the first child her age that she gets along with, but there is something enigmatic about her. Anna struggles with memory and occasionally loses consciousness. What connections do these objects have to Marnie?