Anna Sasaki, a quiet and asthmatic girl, is sent to the countryside to recover her health and spirit. While staying with her relatives by the sea, she spends her days sketching and exploring. One day, she discovers the mysterious Marsh House and meets a cheerful girl named Marnie, who slowly helps Anna open up. As their bond deepens, Anna realizes there is more to Marnie than she first thought. As the summer ends, secrets unfold, leading Anna on a journey of self-discovery and lasting friendship in a season she will never forget.