Fire Island cop Joe DiGregorio goes undercover to find out who killed quietly swinging single Linda Levinson on her way home from legendary nightspot Crane's. Disguised as an accountant(!), Joe D. takes a share in a local house, hunts up Linda's housemate Alison Rosen, finds himself in bed with her after their first evening at Crane's, and wonders how he's going to break the news to his almost- fiancÇe Marie, as he focuses on the three men in Linda's life: her long-standing married lover Jonathan Golland, her last one-night stand Rob Lewis, and Eric Farber, who turns out to be her partner in a securities scam. The plot lurches along rather than thickening; first it looks like Farber, then Lewis, then maybe Golland--no, Farber . . . First-novelist Margolis is best at evoking the intently vacuous ambiance of Fire Island (``I think you're ready for number two on your face''); his characters and mystery, though, are strictly from hunger.