The yearly outing for Spenser. His colleague Hawk has been badly shot up by Ukrainian mobsters. They also kill Luther Gillespie, the bookie Hawk was protecting, and all but one of Luther's family. (This is not a spoiler, it happens before the book starts, and is revealed on page 2.)
Hawk rehabilitates and, naturally, needs to go after the guys who did it. And, being the kind of guy he is, Spenser must help. Complications ensue, because in Spenserland, you can't just find the bad guys and shoot them. That would have been a real short book. Instead, Hawk and Spenser concoct a plan to destroy the Ukranian mob, and set up Luther's surviving child financially. Spenser finds himself in a minor moral delimma, since he usually only kills people in self-defense. So there's a lot of chit-chat about that.
Summary: short on action, short on detecting. Not the best outing.