MacDougal Duff falls in with a strange family -- and senses murder. A sudden cloudburst forces MacDougal Duff to stop his car in front of the home of Mary Moriarty. The history professor turned detective is reclining in his seat, waiting out the torrential rainstorm, when Mary knocks on the car window holding a feverish child and jabbering about a dead woman. After years investigating murders, Mac Duff is not fazed by a dead body, but the sick child moves his heart. He speeds to the hospital, and while the doctor is seeing to the child, he asks Mary about the dead woman. Her name was Brownie, Mary says, and she was poisoned. Unable to resist an interesting death, Mac Duff moves in with the family, pretending to be a distant relation. To his delight, he finds a home corrupted by secrets, whose residents do not hesitate to kill.