Set in New Orleans during the weeks leading up to Mardi Gras, the story follows Caleb Rooney, a New Orleans Police Department (NOPD) detective who also runs a popular food truck called Killer Chef with his ex-wife and business partner, Marlene. During a brunch shift, Caleb spots members of the Franklin Avenue Soldiers, an up-and-coming gang from the St. Roch neighborhood, watching him. His badge and gun are being held by the department pending a review of a fatal shooting.
Caleb walks to NOPD headquarters for a Use of Force Review Board hearing into his killing of Larry Grant, a Franklin Avenue Soldier and drug dealer. Because his double life generated sensational headlines, the department has made the proceedings public. The board questions whether Caleb planted the gold-plated 9mm handgun recovered at the scene; he defends himself with field paperwork documenting an informant's tip about Grant's weapon. As Caleb exits, Grant's younger brother, Ty, shouts a death threat. Caleb visits his former boss, Chief of Detectives Brian Cunningham, and impulsively resigns. That night, Ty nearly runs him over and mimes shooting him through a car window.
Caleb notices an unusual police presence during the first Carnival parade, including Special Weapons and Tactics (SWAT) officers in tactical gear and undercover cops, though none will explain why. When he chases a purse-snatcher through the French Quarter, the pursuit proves a trap: Ty and three Franklin Avenue members beat him with a baseball bat in a parking garage.
Still healing, Caleb and Marlene dine at a restaurant owned by Lucas Bryant Dodd, a Miami restaurateur who pitches a lopsided business deal they decline. When Dodd erupts in rage at a server, his wife, Vanessa McKeon, intervenes. Caleb is drawn to Vanessa, and over the following weeks they develop a deepening connection. She confides that Lucas is controlling and jealous.
The investigation begins when Cunningham arrives at Caleb's house and reveals that Federal Bureau of Investigation (FBI) Special Agent Marcus Morgan briefed senior NOPD officials about a credible terrorist threat during Mardi Gras involving a cell of professional killers. The FBI ordered the NOPD to stay out, and the superintendent complied. Cunningham asks Caleb, now a civilian free from institutional constraints, to investigate independently. Caleb agrees.
He tracks Ibrahim Farzat, a Syrian refugee previously dismissed by Homeland Security, to a secret meeting at a junkyard near the Industrial Canal. After being ambushed there, Caleb learns from Morgan that the FBI had the junkyard under surveillance for months. Morgan orders him to stop. Caleb feigns compliance.
A police report about a suspicious vehicle near a seafood plant connected to the Needham family, a powerful New Orleans restaurant dynasty, shifts Caleb's focus. He meets Billy Needham, David Needham's younger cousin, who reveals that David has made threats of violence and that his half-sister, Emily Beaudette, was an early investor in the café where Farzat worked. Caleb confronts David directly but is forced out at gunpoint by David's armed bodyguard.
Attackers assault Marlene inside the truck one night. Caleb drives to Emily's horse farm in central Louisiana, where she shares ten years of family financial records. Farzat then turns up dead in the Lower Ninth Ward, tortured and with his tongue removed. The records reveal David has been funneling money to Crescent Care, an Islamic charity, through shell companies. Caleb's private investigator friend, Gordon Andrews, reveals that Vanessa has chronic cirrhosis of the liver and that Lucas trapped her in marriage through a prenuptial agreement stripping her of healthcare if she left.
At Crescent Care's office, Caleb identifies the manager, Saleel el-Sharif, by bluffing about having seen his photograph online, then demands documents at gunpoint. El-Sharif produces a hidden shotgun and a firefight erupts. Before fleeing, Caleb finds a photograph of David Needham and Farzat at a charity event, the first concrete link between David and the dead suspect. Following a tip from Cunningham, Caleb explores an abandoned safe house in St. Roch. In the garage, he discovers ammonia, a bomb-making chemical, and tire tracks matching a tractor used to pull Mardi Gras floats. He realizes the terrorists plan to weaponize the parade.
Caleb rescues Vanessa from armed men at a gas station and captures Angus, a white supremacist recruited through Aryan Brotherhood message boards. Angus reveals the cell includes white supremacists, Islamist radicals, and others organized by a "rich restaurant guy." Morgan attempts to arrest Caleb, who flees to a safe house address in the Holy Cross neighborhood, finding only a black light and a Spanish-language radio.
Caleb shares everything with Billy, who calls David "a monster." Billy mentions his flying hobby and how peaceful the world looks from altitude. Caleb then confronts Lucas, exposes how Lucas used Vanessa's medical condition to trap her in marriage, and Vanessa leaves with Caleb.
On Mardi Gras morning, concussion grenades detonate in the French Quarter, designed to panic the crowd into a kill zone. A modified tractor fitted with a spiked grille detaches from its float and plows into civilians. Caleb shoots the driver. A second tractor-float disassembles to reveal NOPD tactical officers secretly placed by Cunningham as a contingency. As costumed shooters fire into the crowd, Caleb discovers the terrorists identify each other using red-green polarized sunglasses that reveal ultraviolet-reflective paint on their costumes, the same paint visible under the black light from the safe house. He writes instructions on a chalkboard and holds it up to an FBI camera, enabling NOPD snipers to neutralize the attackers.
During the chaos, Billy calls and reveals himself as the mastermind. He views the attack as a purge to reclaim New Orleans from outsiders who exploit its culture, citing the government's failures during Hurricane Katrina as justification. He implies more attacks, using the phrase "the sky's the limit," and threatens Marlene and Vanessa.
After three days of rain, a citywide memorial service is scheduled at the Fair Grounds Race Course on the first clear morning, expected to draw 65,000 people including the vice president. A weather report mentions "CAVU" (Ceiling And Visibility Unlimited), a pilot's term for perfect flying conditions. Caleb connects Billy's phrase with his flying hobby and Emily's farm, which has flat pastures suitable for an airstrip. At the estate, he finds Emily shot dead and Billy preparing four Cessna aircraft loaded with explosives and chemical weapons, modified as unmanned drones.
Billy shoots Caleb in the chest, but a Kevlar vest saves his life. Despite broken ribs, Caleb commandeers an all-terrain vehicle (ATV) and races toward the planes as they taxi down a grass airstrip. The ATV flips into the lead plane's tail section, causing a chain-reaction collision that destroys all four aircraft. Caleb shoots Billy as he flees, captures him, but decides not to kill him.
Cunningham later reveals that Russian intelligence promised the FBI information about the plot, causing Washington to delay the investigation in hopes of a breakthrough that never came. Rima Farzat, Ibrahim's widow, reveals her husband had been an FBI informant who discovered Billy's plot and was killed for it. Crescent Care was a legitimate charity, and el-Sharif attacked Caleb to protect his family. The review board clears Caleb in the Grant shooting, and Cunningham offers him his job back.
Two weeks later, Caleb works the truck with Marlene and Vanessa, who is free from Lucas after threatening public exposure of his abuse. Gordon presents Caleb with an application to become a licensed private investigator. Caleb says he will think about it. He and Vanessa walk down Esplanade Avenue, carrying the promise of a new career, a new love, and Caleb's enduring commitment to his city.