The bar in Showman’s Jazz Club, a Harlem destination for visitors from just down the block to Japan and back, stretched from the front door to the stage. The owner, Al Howard, liked to sit at the curve near the entrance.

John Miller, a regular at the club and a deputy commissioner in the Police Department, knew the habit well. “Typical detective thing,” he recalled. “So he could see everyone going in and going out.”

The club’s owner had in fact been a police detective, and the two men became friends. And so, decades later, Mr. Miller was surprised to hear one particular story about Mr. Howard’s years on the force. He wondered if it indeed could be true and, if so, found it shocking that it was not more widely known. So, a couple of years ago, very late one Saturday night — actually, already Sunday morning — after the crowd had thinned and the band had packed up, Mr. Miller took a bar stool beside the club owner and just came out and asked.

“I heard this story that you saved Martin Luther King,” Mr. Miller said.

What happened on Sept. 20, 1958, in a Harlem department store is briefly recounted in history books and old newspaper clippings that dutifully tell the who, what, when and where of a tragedy averted. But lesser known, because it was not in the nature of the men involved to broadcast it, are the snap decisions of a young officer and his partner, dropped into a scene of bedlam and confusion, that would change the course of American history.