The year is 1839, and Mary Shelley - the woman who wrote Frankenstein - is living alone in a tiny cottage on the banks of the river Thames in Putney. As she sorts through the snowstorm of her husband's scattered papers she is reminded of their past: the half-ruined villas in Italy, the stormy relationship with Shelley and her stepsister Claire, the loss of her children, the attempted kidnapping of Claire's daughter Allegra from a prison-like convent in Florence. And finally, her husband's drowning on the Gulf of Spezia as they stayed in a grim-looking fortress overlooking the sea. What she has never confided in anyone is that she has always been haunted by Shelley's drowned first wife, Harriet, who would come to visit her in the night as she slept with her two tiny children in a vast abandoned villa while Shelley was away litigating with lawyers.

Did Mary pay the ultimate price for loving Shelley? Who will Harriet come for next?