After a dinner party, Herbert Kingery accuses one of his guests of defrauding his company. He adds that he will give the criminal until the next morning to confess. That evening, the family dog howls, causing Kingery to open the door of his office to investigate. When Kingery bolts the door, a gun goes off and he is killed. The death is ruled a suicide, but his daughter Gwen is determined to prove he didn't kill himself. Nurse Sarah Keate, who is taking care of Gwen's aunt Lucy, recommends private detective Lance O'Leary. Gwen invites everyone who was present the evening of her father's death to spend a weekend in the country. Shortly after the guests leave for an afternoon of hunting, Helen Page dies in what seems to be another suicide. Helen's husband Joe had recently asked for a divorce to marry another woman. Lance investigates and learns that Helen was strangled and was shot after her death to make it look like suicide. Gwen then visits Gerald Frawley in his room. After she leaves, Gerald removes his toupee and is shot just as Kingery was. Sarah finds a letter Gerald wrote to Gwen, stating that the "key" is in his toupee. Lance believes Lucy Kingery, who is confined to a wheelchair, can actually walk. He tricks her into revealing this and learns that she stole Gerald's toupee, which has the combination of his safe written inside it. As soon as Lance and Lucy leave the room, the family chauffeur rushes in and copies the combination. After ruling out the chauffeur, Lance catches him in the act of opening the safe. The chauffeur confesses that six years earlier he had stolen some money from Kingery's office. Kingery had him sign a confession to ensure that he would stay straight and he wanted to remove it before anyone saw it. Lance discovers that a gun on the wall, hung as a decoration, has been rigged to fire if the office door is bolted. Deciding to trick the murderer, Lance invites everyone into the office. When he asks Julian Barre to bolt the door, Barre punches Lance and escapes but is stopped by the dog. Lance explains that he found a paper in Kingery's safe accusing Barre of forging some securities. Barre spent the money on Helen, who found out about the forgery from Gerald. Therefore, Barre killed both Kingery and Helen as well as Gerald to keep his crimes a secret.