When her husband dies, Cassie Nolan returns home to Ireland, to Inis Mòr, in the windswept Aran Islands. It is the month of Samhain, the festival that marks the beginning and the end of the Celtic year, and announces the start of the Dark Time. As such, Samhain belongs neither to the year that is ending nor to the year that is beginning but is a timeless day when the living may meet the dead.
In the family home, she finds the diary of a young girl, Abby, who disappeared without trace forty years earlier at the same time of year. Cassie will not stop trying to find out what happened to this girl, an obsession that leads her to unearth a family secret that undermines everything she believes in.