During dinner, Sarah’s husband, Arnold, left abruptly, claiming he had urgent work. That night, while watching a basketball game with their son, Sarah saw Arnold on the stadium’s “Kiss Cam”—with Nancy, their ex-nanny. He later texted her, “I can’t talk, I’m at work,” sealing her suspicion with a lie, Heartbroken and furious, Sarah investigated. It was Nancy’s birthday, and she...