Two years after Cal and Dante destroyed the heart of the Boston Irish mob and splintered the Boston political landscape, everyone is out for themselves and trying to claim a piece of the city.

It's early summer but already the city is in the grips of a heat wave, which doesn't help tensions any as the fractures within Boston's underworld are finding their way to the streets in payback shoot-outs between mobsters and the police. When a body is discovered at the Charlestown locks, tarred and feathered and shot, it appears to be a gangland killing, and almost immediately dismissed.

But Cal's cousin, detective Owen Lackey, who intended to intercept a boat coming into Boston weighed down with stolen guns and ammunition, recognises the murder as typical retribution for IRA informers and knows that with this murder there may be much more at stake than his superiors realise. Keen to uncover those responsible, he asks Cal and Dante to help search the Irish-American enclaves of Dudley Square, Dorchester and South Boston for some sign of the killers.

It is a journey that will take them into the shadowy parts of Boston, a world of packed dance halls, Irish wakes and funeral parlors, of down-and-out immigrant rooming houses, bars, and secret meeting halls, and, ultimately, to the discovery of a terrorist plot that will shake the city to its core and bring them head-to-head not only with Cal's past but with the IRA Army Council itself.