The story follows Anna, a young introvert with health issues who is sent to spend the summer with relatives on a quiet island. At first, Anna feels out of place, isolated in an environment that feels foreign to her. However, her life takes an unexpected turn when she discovers a mysterious abandoned mansion located in a nearby marshy area.
The mansion becomes the backdrop for her encounters with an enigmatic girl named Marnie, who seems to be the only one who understands Anna and her feelings of loneliness. As their friendship blossoms, Anna feels more alive than ever and begins to uncover secrets about her past and Marnie's history, which are revealed to be filled with nostalgia and melancholy.
But the relationship between the two girls becomes increasingly complex, and as Anna delves into Marnie's world, she starts to question the reality of their friendship and the meaning of their connection. Marnie's memories are intertwined with a past that could change Anna's life forever, driving her toward a deeper understanding of what family and loss truly mean.
With visually stunning animation and an emotive soundtrack, this film explores themes of love, friendship, and the power of memories, leaving a lasting impression on the hearts of those who watch it.