I walked in on my husband with my two best friends in our bed, but my revenge on them was unforgettable.

My name is Julia, and I’m not the kind of woman who snoops through her husband’s things. But that afternoon, while I was cleaning out the coat closet in the hallway—always a battleground of old receipts and baseball tickets—something slipped out, as quiet and sudden as a slap without sound. A Polaroid photo.