A young girl named Anna Sasaki is struggling with loneliness and asthma in the city. Her adoptive parents decide to send her to a quiet coastal town to stay with relatives, hoping the fresh air will aid her recovery. In this serene environment, Anna explores the rural area and becomes intrigued by an old, seemingly abandoned mansion across a salt marsh. One night, she sees lights in the mansion and later meets a mysterious girl named Marnie. The two quickly form a deep bond, spending their days talking, playing, and sharing secrets. However, Marnie has a strange habit of vanishing without a trace, which leaves Anna puzzled.

As their friendship deepens, Anna starts to uncover clues about Marnie's life and the mansion's hidden history. The line between dreams and reality blurs as Anna becomes determined to understand who Marnie truly is. With the help of new friends and old memories, Anna embarks on a journey of self-discovery that reveals unexpected connections and personal revelations. Through her interactions with Marnie, Anna learns valuable lessons about trust, loss, and love, ultimately discovering more about herself and her place in the world.