Dr. Gillian Hayes undertakes a high-stakes heart operation, hoping to save a patient while a fellow doctor quietly believes the effort may be wasted. When the patient dies, Hayes rails against the decision to abandon the procedure, feeling the loss personally and professionally. The coroner’s report hints at something explosive emanating from inside the chest, a chilling clue that sets off a chain of uneasy questions. Lt. Tom Royko questions Hayes in her office, and she worries that she might be a suspect, though Royko insists she is not.
A nervous patient awaiting a pacemaker worries about what could go wrong during the procedure. Hayes assures him that the operation is routine and safe, joking that she could even perform it blindfolded—an unsettling line given the fear in the room. The patient acknowledges the risk, stressing how delicate heart surgery is. The hospital is then rattled by a phone call from a caller who claims there is a bomb in operating room #4 set to explode at 1:00. The clock shows roughly 12:50 as the receptionist informs the surgical team. Hayes is mid-surgery and refuses evacuation, while a colleague asks if leaving the room is an option. At 12:59, with only Hayes and the patient left, Royko enters to insist she step away. Hayes resists, angry that Royko entered unmasked and potentially contaminated the scene. Royko escorts Hayes out just in time, and the bomb detonates moments later—the device had been implanted inside the pacemaker Hayes had just inserted.
In the aftermath, Royko advises Hayes to assemble a list of ex-partners or others who might be suspects; she learns that she was initially a suspect, but her decisive refusal to abandon the room helps her move off that list. A later clue arrives in the form of a cellular phone found in a Federal Express package at Hayes’s office. The device, linked to the bomber, broadcasts a countdown that correlates with a patient’s heartbeat, suggesting the pacemaker is the trigger. Hayes and Royko hypothesize that the countdown is synchronized to the patient’s vital signs, and that removing the pacemaker without a substitute will cause a fatal explosion. The next victim, Benjamin McDonald, is identified at the airport, and Hayes and Royko rush to save him. They move McDonald to a different room and attempt to remove the pacemaker with a simulator that would trick the device into thinking it is still connected to a heart. When the simulator fails to arrive in time, Royko must operate from a distance, protected by a shield, as the pacemaker is detached and explodes. McDonald’s final words are a plea to his wife and children.
Hayes ultimately learns that the bombs are being planted by Zachary Franklin, whose son died during a prior surgery performed by Hayes. Franklin vows that the next victim will be at an elementary school. Hayes and Royko discover a young patient at a school and rush to intervene. As part of the plan, Lt. Royko uses a fire alarm to draw attention away, while Hayes evaluates the boy’s heart rate. Since the countdown does not accelerate with the heartbeat, Hayes deduces that the timer is not linked to heart activity. The pacemaker is removed, but without a timely simulator, there is no explosion. Royko shields himself behind a barrier as they evacuate the boy, and in the chaos, the teacher who was a patient of Hayes—Jessica Sara—falls victim when her chest explodes as Royko tries to pull her to safety.
Franklin escalates the danger by leaving a birthday cake at Hayes’s home for Sean, prompting police attention and a chilling warning: tomorrow he will kill 14 patients. Hayes and Royko race against time to prevent the new wave of attacks and to protect those at the hospital.
Clues finally point to Franklin’s hideout near Hayes’s office. The real plan unfolds: Franklin has captured Lt. Royko’s son and implants a pacemaker in him, linking that pacemaker to Hayes’s pulse with a specialized device. Franklin then commits suicide by leaping from the hospital roof, taking a bomb with him. Hayes and Royko sprint to the basement in a desperate bid to save the boy—an effort that arrives just in time.
