While a grandfather is unconscious, his son and the butler are murdered; but a nurse helps a police detective solve this mystery based on Mignon G. Eberhart's novel.
Richard Federie (Walter Walker) summoned his relatives Eustace Federie (Hobart Cavanaugh) and Mittie Brown (Dorothy Tree), but his granddaughter March Federie (Patricia Ellis) won't let them see him. Federie gets a telegram that his son Charles is coming. Adolphe Federie (Robert Barrat) arrives and finds his father unconscious. In the rain Sarah Keate (Aline MacMahon) comes to nurse him. Ross Lonergan (Lyle Talbot) arrives and demands that Adolphe pay his loan back. Federie's lawyer Elihu Dimuck (Henry O'Neill) tells Sarah he is waiting to see Federie. Sarah is told by March, Adolphe, Mittie, and Eustace they also want to speak to him first. Adolphe finds a paper in a small elephant and is shot. Sarah finds Adolphe and screams. They call the police, and Lance O'Leary (Guy Kibbee) and Sergeant Jackson (Allen Jenkins) investigate. O'Leary greets Sarah warmly and finds March's slipper. March says she went out to contain the dog. Jackson asks questions belligerently. Isobel Federie (Helen Flint) tells O'Leary her husband Adolphe was cruel. Mittie accuses Isobel of shooting Adolphe. Dimuck is questioned, and March says that everyone hated Adolphe. O'Leary suspects all present, even Sarah, who tells him she found the elephant by Adolphe's body, but now it is gone.
Jackson asks the butler Grondal (Brandon Hurst) to confess. March tells Ross that she knows who did it but can't say. Jackson shows O'Leary Grondal's criminal record. While March sleeps, Grondal gives the elephant to Sarah, who hides it. O'Leary gets a call from Sarah that Grondal was strangled with a violin string. Mittie tells O'Leary that Isobel killed them, because she found the key in Isobel's room. O'Leary finds a revolver in Mittie's drawer. O'Leary tells the press to leave. Mittie screams and says she saw Adolphe. Someone takes the elephant from Sarah, but it is O'Leary. Sarah goes up in the attic and is locked in. O'Leary finds her and tells her of Charles' telegram and release from prison. O'Leary persuades Sarah to stay and gives her the gun to fire at 12:30. When she does, Adolphe's twin Charles is caught. O'Leary explains, and Jackson brings in Dimuck. O'Leary reads Federie's note in the elephant and says Dimuck couldn't give up money he managed. O'Leary sends the family to bed and asks Sarah to marry. Federie wakes up and asks what happened.
Humor lightens this mystery that keeps the audience in suspense guessing which of many suspects committed the murders.