Wren lives in an ancient house on Anglesey overlooking the Menai Strait. The house seems to sing at night as if calling to someone far away, and strange cracks keep appearing in its thick stone walls. Wren lives with her pa, her brother Tudur, and her Aunty Efa who moved into the family home when Wren's mother was killed in an accident that left her aunt paralysed. Wren's pa finds his daughter difficult to control; she's wild like her ma, and dreams of one day being able to fly like a bird.

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When Pa decides to enrol her in the infamous Anglesey Institution for the Re-education of Young Women, Wren sets about building a flying machine to escape the clutches of her oppressive home life. As she begins work with the help of her friend, Medwyn, Wren realises that there is something very, very wrong with her house, and sets about unearthing a horrifying family secret that has been kept deeply buried for generations.