Edgar Marsh, a shy librarian obsessed with erotica, becomes infatuated with his neighbour Betty Clare when he sees her undressing in her bedroom. He invites her to dinner, and although she clearly is uncomfortable with the attention he pays her, he showers her with jewelry and fantasizes about their future. Complications arise when he introduces her to his friend Carl Loomis, whom Betty finds far more attractive and appealing. After witnessing Carl and Betty together in her bedroom, Edgar bludgeons Carl to death with a poker and buries him beneath the floorboards in his piano room. His overwhelming guilt leads him to believe a ticking metronome and the incessant dripping of a faucet actually are the sound of his victim's heart still beating.