Junior high school girl Sasaki Anna (voiced by Takatsuki Ayara) has a lot of worries in her heart, and she can't face her adoptive mother Raiko (voiced by Matsushima Nanako) who loves her. Because of a sudden breathing problem during school, Anna used the summer vacation to come to the beautiful Sapporo City, Hokkaido, to recuperate at the home of her relatives Oiwa Kiyomasa (voiced by Terajima Susumu) and Aunt Ajie (voiced by Negishi Kiyomi). In the stream not far from her uncle's house, there stands an old but unique Western-style building, and the strange feeling makes Anna notice its existence. One night, Anna, who was in a low mood, rowed a boat past the building, and found that the house that should have been uninhabited for many years was brightly lit, and she met a blonde girl of the same age, Marnie (played by Arimura Kasumi). Similar moods made the two girls become good friends. They were attached to each other, but they didn't know what incredible fate was hidden in their hearts... This film is adapted from the children's literature of the same name by British novelist Joan G. Robinson.