TODAY I’M A DETECTIVE with the Iberia Parish Sheriff’s Department. I make a modest salary and live on Bayou Teche with my wife, Molly, who is a former nun, in a shotgun house shaded by oak trees that are at least two hundred years old. With a few exceptions, the cases I work are not spectacular ones. But in the spring of last year, on a lazy afternoon, just about the time the azaleas burst into bloom, I caught an unusual case that at first seemed inconsequential, the kind that gets buried in a file drawer or hopefully absorbed by a federal agency. Later, I would remember the pro forma beginnings of the investigation like the tremolo you might experience through the structure of an airplane just before oil from an engine streaks across your window.

A call came in from the operator of a truck stop on the parish line. A woman who was waiting on a tire repair had gone into the casino and removed a one-hundred-dollar bill from her purse, then had changed her mind and taken out a fifty and given it to the clerk.

“Sorry, I didn’t realize I had a smaller denomination,” she said.

“The hundred is no problem,” the clerk said, waiting.

“No, that’s okay,” she replied.

He noticed she had two one-hundred bills tucked in her wallet, both of them stained along the edges with a red dye.

I parked the cruiser in front of the truck stop and entered through the side door, into the casino section, and saw a blond woman seated at a stool in front of a video poker machine, feeding a five-dollar bill into the slot. She was dressed in jeans and a yellow cowboy shirt. She sipped at her coffee, her face reflective as she studied the row of electronic playing cards on the screen.

“I’m Detective Dave Robicheaux, with the Iberia Sheriff’s Department,” I said.

“Hi,” she said, turning her eyes on me. They were blue and full of light, without any sense of apprehension that I could see.