I take pen in hand to tell the tale of a monster . . .

So begins the narrative penned by eighteen-year-old Prudence Mott. In the summer of 1859, a storm casts a derelict whaling vessel upon the shore of Nantucket. Inside the hold lay twelve mummified bodies of the crew.

Strong-willed Prudence, as unconventional as one can imagine of a teenager in that era, begins an investigation and suspects something terrifying also arrived on the derelict vessel: a horror as old the seas, a primeval creature that sailors have spoken of for centuries.

No one believed them. And no one believes Prudence.

Undeterred, she gathers a former soldier-of-fortune, a Samoan harpooner and a timid scientist to seek out and destroy the 'foul presence' loose on her island.