Once a Detective… introduces Dan Burnett, an NYPD detective who opts for retirement after failing his physical. At fifty-five, he becomes a private investigator, learning the ropes from a P.I. with a similar history. Divorced and living on a sailboat, his one source of happiness is his college-aged daughter.

After assisting his new partner with ongoing cases, he lands one of his own: a beautiful woman whose brother was murdered. With the NYPD abandoning the case, it's up to him to find the murderer, all while grappling with his attraction to the client.

The suspects? Las Vegas casinos, where the brother owed a million dollars, and his second wife, who inherited millions upon his death. Dan doggedly works the case, drawing on his lifelong skills, with the help of a former colleague initially assigned to the case. Tracking a mob hitman leads him on a chase across the country, searching for the truth and ultimately finding it.