Lucy Mitchell's father tells her that "the de Northbys owe you." Her mother dismisses his word as the confusion of a dying man, but Lucy is determined to undertake some investigations of her own. Lucy and her sister Alice find employment as seaweed gatherers in Ravenscar, where she meets Mark Cossart, a handsome army officer. Lucy and Mark have eyes only for each other, much to the chagrin of Zilpha de Northby, who considers Mark her exclusive property. It appears that whilst the sisters are attempting to unravel a mystery of the past, a new dispute may be developing in the present..