No group of law enforcers has fought crime like NYPD detectives. Their tradition went back to tough, brilliant Inspector Thomas Byrnes, named to head up the fledgling Detective Bureau in the 1880s. Byrnes's arsenal included threats, head-knocking and subtle deductions—he once broke a major theft ring by tracing a tiny fiber found at a crime scene. Under Byrnes's flamboyant guidance the detectives in the bureau became known as the Immortals and they dramatically reduced the level of crime in a city as freewheeling back then as the Wild West.

Officer Herman Sachs was a collector of police department memorabilia, and not long before he died he gave his daughter one of his favorite artifacts: a battered notebook actually used by Byrnes to jot notes about investigations.

When Sachs was young—and her mother wasn't around—her father would read aloud the more legible passages and the two of them would make up stories around them.

October 12, 1883. The other leg has been found! Slaggardy's coal bin, Five Pts, Expect Cotton Williams's confession forthwith.

Given its prestigious status (and lucrative pay for law enforcement), it was ironic that women found more opportunities in the Detective Bureau than in any other division of the NYPD. If Thomas Byrnes was the male detective icon, Mary Shanley was the female—and one of Sachs's personal heroines. Busting crime throughout the 1930s, Shanley was a boisterous, uncompromising cop, who once said, "You have the gun to use, and you may as well use it." Which she did with some frequency. After years of combating crime in Midtown she retired as a detective first-grade.

Sachs, however, wanted to be more than a detective, which is just a job specialty; she wanted rank too. In the NYPD, as in most police forces, one becomes a detective on the basis of merit and experience. To become a sergeant, though, the applicant goes through an arduous triathlon of exams: written, oral and—what Sachs had just endured—an assessment exercise, a simulation to test practical skills at personnel management, community sensitivities and judgment under fire.

The captain, a soft-spoken veteran who resembled Laurence Fishburne, was the primary assessor for the exercise and had been taking notes on her performance.

"Okay, Officer," he said, "we'll write up our results and they'll be attached to your review. But let me just say a word unofficially." Consulting his notebook.

"Your threat assessment regarding civilians and officers was perfect. Calls for backup were timely and appropriate. Your deployment of personnel negated any chance the perpetrators would escape from the containment situation and yet minimized exposure. You called the illegal drug search right. And getting the personal information from the one suspect for the hostage negotiator was a nice touch. We didn't think about making that part of the exercise. But we will now. Then, at the end, well, frankly, we never thought you'd determine there was another perp in hiding. We had it planned that he'd shoot Officer Wilkins here and then we'd see how you'd handle an officer-down situation and organize a fleeing felon apprehension."