Aman waits in the darkness, watching what is left of his life slip away. His name is Robert Joel Halderman. At 51, he is a producer for the CBS true-crime series 48 Hours Mystery, the latest post for a battle-scarred newsman who has spent much of his life in war zones. He’s had two divorces, which have saddled him with crushing alimony payments, and his second wife recently sent him “reeling,” as he e-mailed colleagues, when she moved to Colorado with their 11-year-old son, Jimmy.

It’s late August 2009, and Halderman is keeping watch outside his modest house, in Norwalk, Connecticut, as a $100,000 electric Tesla sports car comes to a stop at the end of the road. In the passenger seat is his smart, attractive live-in girlfriend, Stephanie Birkitt, 34, being driven home from work by her boss, David Letterman, who lives 20 miles away on a 108-acre estate in Westchester.

Like the rest of Letterman’s roughly five million viewers, Halderman has watched Dave and Stephanie seduce each other on national TV. For more than 10 years, as Stephanie rose from Late Show with David Letterman intern to one of the host’s assistants, to his frequent on-air sidekick, they bantered together in hundreds of bits. It started innocently enough, with Letterman calling his office from the stage of the Ed Sullivan Theater and asking Stephanie, then 26, about such things as “strippers” and “the dirty little Hooters girls” and her weekend lunch at Super Duper Weenie in Connecticut. “Do you like hot dogs, Stephanie?” he asked. “I love them,” she replied in her adorable schoolgirl voice.

With that, they were off and running, Stephanie coming on the show to taunt, tease, and tantalize him. She would dress in a variety of costumes—leprechaun, Batman, high-school girl in a varsity letter jacket—and what so obviously appealed to Letterman was her brazen disregard for him as a star. “Thanks, Grandpa, bite me!” she would say, playfully punching him, and accusi...