Anna Sasaki, a young girl with frequent asthma attacks, is isolated and unsociable, causing her foster parent great concern. Upon her doctor's recommendation, Anna is sent to the countryside in hopes of improving her health and mental state with cleaner air and a more relaxed lifestyle. She spends her summer days living with her aunt and uncle in a small town near the sea, indulging in her passion for sketching. During one of her walks, Anna stumbles upon an abandoned mansion called The Marsh House. But the place is not unoccupied. She meets a mysterious girl named Marnie, whose joyful nature gradually brings Anna out of her shell. The two form a strong bond, but Anna soon realizes that Marnie is not everything she seems to be. As her stay in the town nears its end, Anna uncovers the secrets of The Marsh House and the true identity of Marnie. Omoide no Marnie is a heartwarming tale about a young girl's journey of self-discovery and friendship. It portrays the unforgettable summer that Anna will cherish in her memory forever.