An NYPD detective is always on duty. Erin O'Reilly is just settling down for a drink with her favorite gangster, celebrating her promotion to Detective Second Grade, when she hears the call every cop dreads: "Officer down." Soon Erin and her K-9 partner Rolf are chasing a cop-killer through--and beneath--New York City, racing to stop a lunatic's plan to tear the heart out of her city. To make matters worse, rumor has it there's a dirty cop in Erin's precinct, and the watchdogs at Internal Affairs are sniffing around Erin. Now she'll have to prove herself like never before as terrorists, assassins, and even allies stand in her way as she fights to save Manhattan