Corbett and his companions have narrowly escaped the island of Golgotha and are recuperating in the town of Alghero on the northwestern coast of Sardinia, currently under Spanish rule. Hudson Greathouse is at his limits, both mentally and physically, after their harrowing ordeal.

Their respite is short-lived when the Spanish authorities learn of their abandoned quest for Brazio Valeriani and the mirror created by his father, Ciro. This mirror is rumored to have the power to summon demons from the underworld, and the Spanish authorities are eager to possess it. Consequently, a professional witch-hunter and soldiers are assigned to accompany Matthew, Hudson, Professor Fell, and Cardinal Black as they resume their quest.

Their only lead on Brazio's whereabouts takes them to Venice. Unbeknownst to them, members of an established crime family also seek the mirror for their own nefarious purposes, and a betrayal from within Matthew's cohorts puts the family on their trail. The journey presents numerous challenges, including a close encounter with the French-Dutch war, forcing Hudson to confront personal demons from his past and putting the entire group in grave danger.

As they race towards the mirror with multiple competing agendas for its future, Matthew finds help and support from unexpected places. The quest will push him to his limit once again, cause him to question his own beliefs about what is real and what is fantasy, and ultimately what his future holds.

Leviathan is the final installment in bestselling author Robert McCammon's acclaimed series of historical thrillers featuring Matthew Corbett, a brave and resourceful "problem-solver." Matthew aims to solve the mystery of the mirror once and for all and end his entanglement with Professor Fell, so he can return to New York to start his life with Berry Grigsby. However, this ultimate test of his wits, cunning, and ingenuity might prove to be more than even Matthew Corbett can handle.