Fifteen-year-old Wulliam is preparing to unwillingly take up his family's mantle as Riverkeep, tending the river and fishing corpses from its treacherous waters. But when his father is possessed by a dark spirit, Wull hears that a cure for Pappa's illness lurks in the belly of a great sea-dwelling beast known as the mormorach, and so he embarks on an epic journey down the river that his family has so long protected - but never explored.