Anna Sasaki is a 12-year-old girl with low self-esteem living in Sapporo with foster parents, Yoriko and her husband. One day, she suffers an asthma attack at school. At the doctor's recommendation to send Anna to a place where the air is clean, her parents decide to have her spend summer break with Yoriko's relatives, Setsu and Kiyomasa Oiwa, who live in a rural seaside town located between Kushiro and Nemuro.

Anna investigates an abandoned mansion across a salt marsh. She finds it familiar but gets trapped by the rising tide until she is found by Toichi, an old fisherman. Anna sees a blonde girl in the mansion. On the night of the Tanabata festival, she meets the girl, Marnie. The two agree to keep their meetings secret. Marnie invites Anna to a party at the mansion, where she sees Marnie dancing with a boy named Kazuhiko.

Anna meets Hisako, an older woman who paints. Hisako comments that Anna's sketches look like a girl whom she knew when she was young. A family moves into the mansion. During the move-in, Anna meets a girl named Sayaka, who gives her Marnie's diary that had been hidden in a drawer. Anna tells Marnie she found documents that show her foster parents are paid to take care of her. She makes the assumption that they only pretend to love her for the money, and says she can?t forgive her biological family for leaving her behind and dying. Marnie shares how her parents are always traveling abroad, and how she is left behind with her cruel nanny. The maids bully her and threaten to lock her in the silo near the mansion. Anna leads Marnie to the silo to confront the latter?s fear of it. Marnie conquers her fear and Kazuhiko comforts Marnie.

Sayaka finds the missing pages from Marnie's diary, which include passages about Kazuhiko and the silo. She and her brother find Anna unconscious with a high fever. They bring her back to the Oiwas, where Anna confronts Marnie. Marnie says she is sorry for leaving her and that she cannot see Anna anymore. When Anna recovers, Hisako reveals Marnie's story: Marnie married Kazuhiko and had a daughter named Emily, but he died from a sudden illness and Marnie committed herself to a sanatorium to cope with her loss. With no other family to care for her, Emily was sent to a boarding school. Marnie recovered but preteen Emily was resentful for her mother abandoning her. In her adulthood, Emily ran away from home and had a daughter herself, but she and her husband were killed in a car accident. Marnie raised her granddaughter, who was placed in foster care after her death.

At the end of the summer, Yoriko arrives to take Anna home and is delighted to see Anna having made new friends in Hisako, Toichi and Sayaka. She gives Anna a photograph of the mansion and says it belonged to Anna's grandmother. When Anna sees Marnie's name written on the back, she realizes that she is Emily's daughter and Marnie's granddaughter. This revelation brings closure about her identity. Yoriko tells Anna about the government payments, but reassures her that they have always loved her. For the first time, Anna calls Yoriko her mother.

During the end credits, Anna says goodbye to her friends and promises to visit again next summer before seeing Marnie at the mansion window, waving goodbye to her.

https://en.wikipedia.org/wiki/When_Marnie_Was_There
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