1759, Ipswich. Sisters Peggy and Molly Gainsborough are the best of friends and do everything together. They spy on their father as he paints, they rankle their mother as she manages the books, they tear barefoot through the muddy fields that surround their home.

But there is another reason they are inseparable: from a young age, Molly has had a tendency to forget who she is, to fall into mental confusion, and Peggy knows instinctively that no one must find out.

When the family move to Bath, the sisters are thrown into the whirl of polite society, where the merits of marriage and codes of behaviour are crystal clear, and secrets much harder to keep. As Peggy goes to greater lengths to protect her sister from the threat of an asylum, she finds herself falling in love, and their precarious situation is soon thrown catastrophically off course. The discovery of a betrayal forces Peggy to question all she has done for Molly – and whether any one person can truly change the fate of another.