Fifty prostitutes have gone missing without a trace from the streets of Boston over four years, and nobody seems to care.

The police aren’t interested because there are no eyewitnesses and no bodies – hence no suspicions of any crime.

Then, a young art student’s grandparents in Kittery, Maine, start posting fliers about their missing granddaughter, and retired Boston police detective Anne Bouchard lobbies Mike Houston, her former partner and now significant other, that they should get involved. Against his better judgment, Houston, trying to live a quiet life with Bouchard in a cabin in the Maine woods, agrees to meet the grandparents.

This is the setup for Vaughn C. Hardacker’s thriller, “The Fisherman,” the second in his series featuring Bouchard and Houston.

Though they’re now private investigators, Houston seems more prone to want to split wood to heat their cabin, while Bouchard can’t resist the urge to end the anguish of two kindly grandparents.

It becomes clear to the detectives from the photo the Kittery couple shows them that Cheryl Guerette is likely a prostitute strung out on heroin and that her “agent” is her pimp and drug supplier.

The reader is introduced early to the dark netherworld that Cheryl – known as Cheri on the streets – inhabits. What is revealed as the story builds is the horror of the even darker world she has fallen into as a captive of Richard Fisher.

Fisher carries on conversations with his dead father and his demented, mute, bedridden mother. Ugly, deformed and chillingly psychotic, Fisher has been abducting a string of women to find one his mother will finally approve of so he can breed an heir to his sinister kingdom. The captives who had preceded Guerette all failed to win “Mum’s” approval and met fates of the grimmest kind.

There is a tangential subplot that involves Jimmy O’Leary, a mob boss who controls illicit activity on the docks around Boston Harbor. O’Leary and Houston were childhood friends who grew up together in South Boston, and though Houston is reluctant to reach out to him, Bouchard knows that O’Leary’s knowledge and connections to the underworld could be a resource for them in their hunt for Guerette.

In concept, bringing O’Leary into the story offers rich potential. But Hardacker lets O’Leary’s presence morph into a storyline that, though it is loosely linked in theme to the missing streetwalkers, diverts energy that could have been better applied to more robust plotting and development of the central story.

“The Fisherman” falls into the darkest corner of the thriller genre. Even still, it could have been a better book if Hardacker’s editor had pushed him to tighten the storyline.

That said, I’m intrigued to know how the author might develop Bouchard and Houston in future stories.

Frank O Smith is a Maine writer whose novel, “Dream Singer,” was named a Notable Book of the Year in Literary Fiction in 2014 by Shelf Unbound. It was also a finalist for the Bellwether Prize. Smith can be reached via his website: frankosmithstories.com.