This is the fourth entry in Tana French's "Dublin Murder Squad" series. The protagonist narrator here is Mick "Scorcher" Kennedy, a supporting character in the previous book, Faithful Place.
Mick makes it clear from the get-go that he's nobody's sweetheart. He's all about the job, corralling the perps, being the best detective on the squad. He's got no patience for fools, and he thinks just about everyone is a fool. For a new and horrific case, he takes on a new partner, Richie. Not because he'd like to show the youngster the ropes and act as a benevolent mentor; instead, he thinks that the youngster will be easier to browbeat into doing things Mick's way.
But the case really is horrifying. "Broken Harbor" is the old name for a new real estate development on Ireland's east coast, and it's only partially finished because the developers have gone bust. But one of the finished houses has been the scene of a gruesome attack, leaving a dead husband, two dead kids, and a wife in intensive care. And there's some really weird shit at the crime scene, involving a lot of baby monitors, holes punched in the walls, an open attic hatch with wire mesh over the opening.
As the case develops, it turns out Mick (unfortunately) has Broken Harbor history that shook his own family, and threatens his ability to deal with the present situation. As has been the case with the previous books in the series, the story is not just about solving the case, but also the psychic damage that the solution wreaks on the participants.