Lake Superior has called Mary Whitney once more to its shores. While her husband, Michael Tanner, finishes several projects in Seattle, Mary flies alone to settle in to her chartered sailboat in Bayfield, Wisconsin. She is looking forward to a relaxing solo sail and a few days communing with the lake. 

Neither she nor Michael are aware that Edwin Tobias, a wealthy and determined hedge fund operator and corporate raider–not to mention Mary's twisted ex-husband–has been tracking her movements. 

With a yen for intrigue, Mary intends to visit most of the Apostle Islands and being on her own allows her that blissful freedom. Shrouded in mystery, the island that draws her highest interest, aptly named Devils Island, is isolated and lacking a protected anchorage. No one lives on the island and it is rarely visited. Its dramatic history and unceasing wave action has left the formation great caves that emit sonorous moans and booming sounds. It is there, off that rocky coast, Mary meets the ultimate challenge of her life. 

Will all her mental and physical resources be enough to save her life and protect her boat?