Some politicians are corrupt and will stop at nothing to be elected.

This is the unsurprising premise of the unsurprising “Broken City,” whose acting ensemble deserves better than this mediocre screenplay. It’s also surprising that the talented Allen Hughes (half of the Hughes Brothers, who directed “Menace II Society” and “From Hell”) was attracted to this project.

The show begins with Billy (Mark Wahlberg), a New York City police detective, defending himself against a charge that he killed someone. Mayor Hostetler (Russell Crowe) tells Billy he’s a hero while Police Chief Carl Fairbanks (Jeffrey Wright) remains skeptical. Billy ends up resigning from the force.

Seven years later, he’s a private investigator with an actress girlfriend (Natalie Martinez). Billy’s tough, determined assistant Katy (Alona Tal) helps his business stay afloat.

Hostetler calls Billy one day because he needs someone to gather evidence that his wife Cathleen (Catherine Zeta-Jones) is cheating on him. Hostetler wants to know the man’s identity, and he also wants pictures for proof. Hostetler is vying against Jack Valliant (Barry Pepper) in a mayoral race, and he doesn’t want to lose the election because of his unfaithful wife.

Meanwhile, Billy has problems of his own in the love department. His girlfriend is starring in an independent film, and Billy thinks the director has eyes for her. This leads him to consider taking a drink for the first time in years. Also, Cathleen is sharp enough to know that Billy is following her — and that he is being manipulated.

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There are a couple of “reveals” toward the end of the movie, but they’re thrown in with such a cavalier approach that one of them almost goes unnoticed. The other ends in a fizzle and is far from astonishing.

The cast is interesting, although Zeta-Jones really doesn’t have much to do. Wahlberg is solid as a private eye, and Crowe always gives heft to his characters with charisma and confidence. I have always liked Barry Pepper, so it’s nice to see him in this top-notch cast as a character who struggles with guilt and secrets.

This is far from a candidate for the worst movie of 2013. On the other hand, there won’t be any Oscar nods for it, either. As my Grandma Ryan used to say, “It’s not broken, but it’s badly bent.”