John Ferguson, a whistleblower at Denning, a major pharmaceutical company, becomes disillusioned with the perceived lack of action against the firm. He murders several executives and then ends his life as the police arrive. Tom Miller, Max Minghella, a paralegal assigned to the Denning case, soon questions the strength of the upcoming trial since Ferguson was their star witness. Miller’s boss dismisses his idealism as financially reckless, and when Miller pens an unsolicited memo about the Gambizzi crime family, Alan Emmerich, one of the partners at the firm, fires him. Roger, Miller’s friend and fellow paralegal, initially shares his frustration but stays to work late. Emmerich sends everyone else home due to a supposed technical problem with the firm’s Internet access, including Miller’s girlfriend Anna, Eloise Mumford.
As Miller leaves the building, he sees a man in a suit set down a briefcase. A different man then picks up the briefcase and steps into an elevator. Curious, Miller follows and watches him pull a pistol from the case and shoot Janine, a lawyer. Janine, Molly Hagan, is killed, and Miller hides in a bathroom to catch his breath, discovering the security guard’s body. He warns Roger about the danger, but Roger objects to Miller’s plan to protect Emmerich, arguing that Emmerich wouldn’t do the same for them. Undeterred, Miller urges Roger to call the authorities as he hunts for Emmerich. Before Roger can act, the killer finds him and strangles him, leaving Miller to grapple with the unfolding crisis as his recharging cell phone chimes and reveals that there are more people in the building than the killer anticipated.
Miller leverages his knowledge of the building’s security to maneuver through corridors and finally locates Emmerich in an office with a woman identified as Lorraine Gambizzi. Lorraine, Marina Black, tries to defend herself with a pistol when the killer bursts in, but the hitman shoots her first. Blindsided, Emmerich survives but is forced to stay in the room as the killer departs. Miller surmises that the killer is a Gambizzi hitman sent to silence Lorraine, who was poised to testify against her husband. With Lorraine dead, Miller hopes the killer will retreat, but the target presses on, attempting to gain entry to the record room—a reinforced vault the two use as a safe room. The killer taunts Miller by drawing Anna back to the building with the help of a recharged cell phone, pressuring Miller to surrender the access card.
The killer seizes Anna as she arrives and removes any chance of negotiation. Miller tries to trade his access card for Anna’s life, but the killer reneges and wounds Emmerich in the process. With access to the records gained, the killer starts a fire, only to realize that Miller has duped him with a fake card. Trapped in the vault as the flames burn, the killer chokes and loses consciousness. Miller re-enters to extinguish the blaze, and when the killer regains consciousness, Anna knocks him out with a fire extinguisher. Emmerich recovers the killer’s pistol and fatally wounds him before dying himself; as he breathes his last, the killer whispers a code to Miller, hooking him into the deeper motive behind the carnage.
Driven by this revelation, Miller investigates the killer’s last words and uncovers the likelihood that Emmerich targeted Roger for assassination because of his own whistleblowing about the law firm’s plans to manipulate the Denning case. Confronting Emmerich with this knowledge, Miller learns that the partner begs him to destroy the evidence, arguing that the pharmaceutical industry is more powerful and ruthless than any crime family. Miller refuses, and Emmerich commits suicide. As Miller and Anna leave the building, the killer’s accomplice—a police officer—signals his superiors, while a hitman outside the building confirms to his own team that Miller is now a person of interest. The case closes with Miller stepping into a tense, uncertain future, the city’s shadows lingering as the truth behind Denning’s secrets continues to ripple outward.
