Plum Island

John Corey, a mid-forties NYPD homicide detective recovering from three gunshot wounds, is convalescing in his uncle's farmhouse in Mattituck, on the North Fork of Long Island. One September evening, Southold Township Police Chief Sylvester Maxwell arrives to tell Corey that Tom and Judy Gordon, a married couple Corey knew socially, have been found shot dead on the back deck of their waterfront home. The Gordons were Ph.D. biologists who worked at the Plum Island Animal Disease Center, a secretive federal facility off the North Fork where dangerous animal pathogens are studied. Max hires Corey as a consultant and brings him to the crime scene.

Corey examines the scene. Each victim has a single close-range gunshot wound to the head. The house has been ransacked, but the staged burglary is unconvincing: Expensive electronics remain untouched. The Gordons are wearing dark clothing and running shoes rather than their usual boating attire, with distinctive reddish-brown soil embedded in their shoe treads. A large aluminum ice chest always kept on their speedboat, the Spirochete, is missing, and the boat is only temporarily tied to the dock, suggesting they planned to go out again.

Detective Beth Penrose of Suffolk County Homicide arrives and clashes with Corey before they reach a working truce. That night, FBI agent George Foster and Ted Nash, who claims to represent the Department of Agriculture but is later revealed to be CIA, join the investigation. Nash raises the possibility that the Gordons stole biological agents from the island and were killed during a deal gone wrong. Corey privately doubts this, considering the Gordons incapable of such an act.

Working through the night with the Gordons' financial records, Corey discovers the couple paid $25,000 for an acre of undevelopable bluff overlooking Long Island Sound. In their navigational charts, he finds the number "44106818" penciled near a page showing Plum Island. He confides to Beth an alternative theory: The Gordons may have been drug runners, given their expensive speedboat and late-night trips.

The next morning, the group ferries to Plum Island. Corey arrives early enough to spot Nash and Foster returning from a pre-dawn visit, confirming they have already coordinated a cover story. Security chief Paul Stevens tours them through the island's abandoned military fortifications and the research facility, noting the Gordons were amateur archaeologists who dug around the island and liaised with the Peconic Historical Society. Inside the labs, the island's director, Dr. Karl Zollner, reframes the case: He argues the Gordons stole not a pathogen but an experimental Ebola vaccine worth millions. Everyone outwardly accepts this theory. The federal presence is withdrawn, leaving Beth as the sole investigator. Privately, Corey and Beth agree the vaccine theory is a constructed lie, but they cannot yet identify the true motive.

After Max terminates Corey's consulting role under political pressure, Corey continues investigating alone. He visits Fredric Tobin, a wealthy local vintner who owns an elaborate winery topped by a five-story tower. Tobin claims only casual acquaintance with the Gordons, but when Corey shows a photo to Edgar and Agnes Murphy, the Gordons' elderly neighbors, both identify Tobin as a bearded man who visited Judy Gordon in a white sports car, contradicting Tobin's denial.

At the Peconic Historical Society, Corey meets its president, Emma Whitestone, an archivist trained at Columbia University. Emma reveals she was Tobin's lover and that Tobin actively cultivated the relationship with the Gordons, contrary to his claims. She discloses that Tobin is deeply in debt and loses heavily at the Foxwoods casino in Connecticut. Corey and Emma become romantically involved. One evening, floating in the bay after a swim, Corey experiences a flash of insight when Emma mentions Captain Kidd's Trees, a local legend about buried pirate treasure. A cascade of connections follows: The archaeological digs on Plum Island, the Peconic Historical Society membership, the purchase of bluffs known locally as "Captain Kidd's Ledges," the number 44106818, historically documented as a number the privateer Captain Kidd wrote to his wife from jail, and a Jolly Roger flag the Gordons flew all point to one explanation. The Gordons were not stealing germs or running drugs. They were digging up Captain Kidd's buried treasure.

Emma provides history of William Kidd, a 17th-century privateer who buried portions of his plundered fortune around Long Island Sound before his arrest and execution in London in 1701. Corey reasons that treasure was buried on Plum Island, known as "Pruym Eyland" on old Dutch maps, and that the Gordons discovered it through their archaeological work. Since anything found on federal land belongs to the government, they planned to move it to their own property and announce a legitimate discovery. Tobin, who somehow obtained a document pointing to the treasure, recruited the Gordons because he had no access to the restricted island. Desperate for money and unwilling to share, Tobin planned from the beginning to kill them.

The case turns violent when Beth delivers devastating news. Emma has been found beaten to death with a fireplace poker, with evidence of sexual assault, the scene staged to look like a burglary. The Murphys have also been murdered in their bed. Corey channels his grief into resolve: "The murderer is Fredric Tobin." Beth shares forensic findings, including Tobin's registered Colt .45 automatic and phone records showing frequent contact with the Gordons.

As Hurricane Jasper bears down on Long Island, Corey and Beth search Tobin's properties. In his tower apartment, Corey finds a forged parchment map showing treasure on Founders Landing, Tobin's waterfront estate. In the basement wine cellar, they discover the Gordons' missing ice chest containing a human skeleton with copper coins in its eye sockets, remnants of an original treasure chest, and four gold coins, but not the main hoard. The housekeeper reveals Tobin has departed by boat with rifles and a shovel.

Corey takes the keys to a Formula 303 speedboat in Tobin's boathouse and pursues him through the hurricane, with Beth jumping aboard. Tobin's radar-equipped cabin cruiser overtakes them repeatedly as rifle fire is exchanged in towering waves. Beth is grazed by a bullet and fires a signal flare into Tobin's bridge, briefly setting it ablaze. With fuel nearly gone, Corey drives through the Plum Gut, the treacherous tidal channel near Plum Island, during the storm's eye and beaches the shattered speedboat on the island's shore.

On the island, Corey tracks Tobin inland through Fort Terry, the abandoned military complex, to underground ammunition magazines beneath the old artillery fortifications. In a pitch-dark magazine, the two confront each other. Tobin reveals that the Gordons anticipated his betrayal and moved the treasure, leaving a note saying its location would never be rediscovered. He confirms killing Emma as retribution against Corey. Tobin fires and misses; Corey charges him in the darkness, overpowers him, and leaves him gravely wounded.

Climbing out through an artillery emplacement, Corey is confronted by Stevens, who reveals he followed the Gordons' every move, found the treasure after they relocated it, and intends to kill Corey. Beth, who tracked Corey through the storm, shoots Stevens dead from behind, saving Corey's life. They shelter through the night and take Tobin's boat to the mainland in the morning.

Back in Manhattan, Corey finds a posthumous letter from the Gordons, mailed before their deaths, confirming the Captain Kidd treasure story, Tobin's involvement, and their fear he would kill them. Tobin survives and is jailed awaiting trial for multiple murders. Stevens takes the treasure's location to his grave. Corey negotiates his retirement from the NYPD and takes a teaching position at John Jay College of Criminal Justice. Months later, on the first night of his new semester, Beth Penrose appears as an enrolled student in his class. She has bought a weekend cottage in Cutchogue and made the long commute to reconnect with him, and the novel ends with the two beginning to rebuild their relationship.