Underneath all the distractions, there’s a good little mystery in James Patterson’s “4th of July” (Little, Brown, $27.95), written with Maxine Paetro. But a complex thriller it’s not.

As the story opens, San Francisco Police Lt. Lindsay Boxer is awaiting trial on brutality charges. She killed one teen and paralyzed another after the duo, a sister and her brother, shot and wounded Lindsay and another officer.

The family is suing Lindsay for wrongful death, excessive use of force and police misconduct, which could ruin her career. She flees the pretrial frenzy for the serenity of Half Moon Bay, 40 minutes south of San Francisco, where her sister lives.

But instead of relaxing on the sun-porch of her sister’s house or running on the beach with her dog, Martha, Lindsay finds herself drawn into a series of grisly murders.

The victims have been slashed and whipped. There is no apparent motive. Lindsay wonders if the murders are connected to an unsolved case of hers.

She is out of her jurisdiction, and the town’s police chief wants her to stay out of his investigation. But as the bodies pile up, he calls on her for assistance.

Lindsay, driven by her detective’s instincts and the need to solve a 10-year-old murder, becomes a target of the killers, who riddle her sister’s house with gunfire as Lindsay and Martha huddle inside. Lindsay had been working in her nieces’ room. Her gun is in the living room, and so is the nearest phone.

“How could I be so vulnerable?” Lindsay fumes. “Was I going to die trapped in this room? My heart pounded so hard it hurt.”

Lindsay and Martha survive the attack. When the police arrive, they find a man’s belt with what appear to be blood stains on the metalwork. A clue – or a warning?

Any sane person would have packed up and found safer lodging, but Lindsay refuses. “This is my sister’s house. I’m not going to leave.” (Besides, she has to find someone to repair the damage before her sister returns from vacation.)

When Lindsay isn’t on trial in San Francisco, visiting crime scenes at Half Moon Bay, or drinking and dining with her friends in the Women’s Murder Club, she’s busy restoring an old car and feeding her sister’s pet pig. The car is an ’81 Pontiac Bonneville and the pig is named Penelope, otherwise known as plot devices.

Although “4th of July” is billed as the fourth and latest installment of the Women’s Murder Club series, there is little for the other club members to do. The mystery is intriguing, but the plot is convoluted (one can safely assume that Lindsay will be acquitted at trial). The dialogue and narration are uneven, leaving Patterson’s many fans wishing he’d written the book alone.

Hats off to Martha, a border collie, and the smartest character in the novel.