A Boston police officer goes to Nantucket to investigate the murder of his partner, but he finds more than he bargained for.

When 4 year old Amanda McCready disappears from her home and the police make little headway in solving the case, the girl's aunt, Beatrice McCready hires two private detectives, Patrick Kenzie and Angie Gennaro. The detectives freely admit that they have little experience with this type of case, but the family wants them for two reasons—they're not cops and they know the tough neighborhood in which they all live.

To take down South Boston's Irish Mafia, the police send in one of their own to infiltrate the underworld, not realizing the syndicate has done likewise. While an undercover cop curries favor with the mob kingpin, a career criminal rises through the police ranks. But both sides soon discover there's a mole among them.