When her asthma takes a turn for the worse, 12-year-old tomboy Anna (voice of Hailee Steinfeld) is sent by her foster mum to stay with relatives in a seaside town in the hope that the clear air will help Anna's condition. While there Anna sees an old, abandoned mansion across a salt marsh, but on repeated visits finds the building to be in pristine condition and learns that Marnie (Kiernan Shipka), a young blonde girl, lives there.
The two girls become friends but the lines between Anna's reality and her dream world become blurred, leaving her confused about who Marnie really is. As she attempts to discover the truth about her new friend, Anna also makes some discoveries about herself...
