Moonlight Mile, By Dennis Lehane

We expect the private eye to act a little like God, punishing the guilty and rescuing the virtuous – that is the brief writers like Hammett and Chandler gave their creations. Flawed men and women they may be, but we expect that, by story's end, they will do the right thing.

But what if there is no right thing? What if all possible choices are bad? Dennis Lehane made his name with thrillers set in South Boston, featuring business and sexual partners Angie Rizzo and Patrick Kenzie. The fourth of these – Gone Baby Gone – was competently filmed by Ben Affleck. Other non-series thrillers by Lehane – Mystic River and Shutter Island - were filmed by Clint Eastwood and Martin Scorsese. Fine as his early books were, his move towards psychological thrillers, and to portraits of a community in both Mystic River and The Given Day, make his decision to go back to his series characters surprising – though, in the event, smart and necessary .

At the end of Gone Baby Gone, Patrick hands kidnapped child Amanda back to her mother. But the mother is feckless and the kidnappers, who serve long jail terms, had the most moral of intentions. His relationship with Angie collapsed and only gradually recovered. Here Amanda, now a bright teenager, disappears again and Patrick is threatened by criminals to keep him from showing an interest.

These days he has something to lose – he and Angie have a child and he is struggling to keep his firm above water, working as a sub-contractor for an agency that cleans up messes for rich people. Psychotic Russian gangsters are on Amanda's trail: men handy with acetylene torches as well as guns. Patrick took responsibility for Amanda's life 12 years ago, and his belief that he did the right thing then makes him determined to do it now.

Moonlight Mile is an excellent thriller because it takes its competent hero and heroine and puts them out of their depth. The very integrity with which they approach life – and their sense that victims and villains can be told apart – makes them vulnerable. Lehane is as skilful as ever with plot, action and an evocative sense of Boston's suburbs, but the real strength of this coda to a series lies in the way it interrogates and subverts the values on which the books – and their genre – are built.

Who, in the end, makes the private detective God? And how is his sense of mission to be distinguished from the sense of entitlement of a seedy lecher and a self-help guru, or the Machiavellian drive of an ambitious mobster? The consensual reality of this sort of thriller – Lehane seems to be arguing – can only take too much contact with serious thought.

Roz Kaveney's 'Superheroes!' is published by IB Tauris