One of Sarah's daughters died. But can she be sure which one?

A year after one of their identical twin daughters, Lydia, dies in an accident, Angus and Sarah Moorcroft move to the tiny Scottish island Angus inherited from his grandmother, hoping to put together the pieces of their shattered lives.

But when their surviving daughter, Kirstie, claims they have mistaken her identity--that she, in fact, is Lydia--their world comes crashing down once again.

As winter encroaches, Angus is forced to travel away from the island for work, Sarah is feeling isolated, and Kirstie (or is it Lydia?) is growing more disturbed. When a violent storm leaves Sarah and her daughter stranded, they are forced to confront what really happened on that fateful day.