Scotland 1373: Crispinus, the head of Oronsay Priory, is found dead, strangled and battered, his mouth stuffed with sand, in the middle of the tidal strand separating the holy island of Oronsay from the larger island of Colonsay. The Lord of the Isles, overlord of the islands, assigns the task of finding the killer to Muirteach, the Prior's bastard son.
Since breaking with his father and leaving the monastery, Muirteach has spent his time acting as scribe for his uncle, an island chieftain. Muirteach's father, a powerful churchman, leaves behind a mistress and several other bastards as well as his embittered eldest son, who is thankful he will never hear the words, "My bastard son, the cripple" again.
As Muirteach seeks to find his father's murderer he is helped by the physician Fearchar Beaton, and his daughter Mariota. Muirteach himself is suspected of the murder, as are the Prior's mistress, her family, and the sub-prior. The killer strikes again, His Lordship wants the mystery solved before the Pope and the King in Edinburgh learn of it, and Muirteach himself must come to terms with his own troubled relationship with his father. Muirteach's investigation submerges him deep into a whirlpool of deceit, long buried sins, and treachery, from which no one emerges unscathed.