Here is one possible plot for a murder mystery about a down-on-his-luck private investigator:

In the city of Los Angeles, a struggling private investigator named Danny Gumbois approached by a wealthy businessman named Edward Jones. Jones hires Jack to investigate the murder of his wife, who was found dead in their mansion.

At first, Jack is skeptical of Jones's innocence, but as he begins to dig deeper into the case, he uncovers a web of lies and deceit. He learns that Jones's wife was having an affair with a young actor, and that Jones had hired a hitman to kill her.

As Jack delves deeper into the case, he becomes the target of the hitman, who is determined to silence him before he can reveal the truth. Jack must use all of his skills and cunning to stay one step ahead of the hitman and solve the case before it's too late.

In the end, Jack confronts Jones and the hitman in a tense showdown, and manages to bring them to justice. His success in the case restores his reputation and revitalizes his career, and he becomes a respected and successful private investigator once again.