Detailed Explanation
Cora Tannetti's murder of Frankie Belmont—a stranger on a beach—seems random until Detective Ambrose uncovers her suppressed trauma. Years earlier, Cora witnessed her sister Phoebe's death during a drug-fueled party. She was drugged and assaulted, and her mind blocked the memories. When Frankie played "Huggin and Kissin" on the beach—the song that was playing during her trauma—everything came flooding back. In that moment, Cora saw not Frankie but the men who hurt her sister. The murder was both inexplicable and tragically inevitable.