Private detective Michael Shayne is a juror at the trial of Lillian Hubbard, a socialite accused of murdering her fiancé, Harley Forsythe. One afternoon, Lillian's attorney, John Logan, calls to the stand Henry Randolph, a butler who works in the apartment next door to Lillian's. Randolph testifies that he saw Lillian arrive home at 7:15, although a previous witness, singer Rita Darling, testified that she saw Lillian at Forsythe's apartment at 9:30, which means that she was there in time to commit the murder. Before Randolph, who is visibly nervous, can be cross-examined, a mysterious man enters the courtroom, throws a knife at Randolph and kills him. Shayne hides the knife, then later that evening, sneaks out of the hotel in which he and the other jurors are sequestered, and returns to the courtroom to get the knife. It has already been retrieved by Judy Taylor, however, a reporter and Shayne's ex-girl friend. When Judy insists on going along while Shayne investigates, he explains that he suspects that someone killed Randolph in order to prevent Lillian from being acquited. At Logan's apartment, Shayne deduces that Randolph could not have seen or heard Lillian as he claimed, which means that the butler perjured himself. Shayne and Judy then go to a vaudeville theater at which knife thrower Count Edmond Telmachio is performing, and in his dressing room find a knife identical to the one used to kill Randolph. Telmachio knocks out Shayne and flees, but Shayne and Judy follow him to a warehouse. There, they discover the body of the recently strangled Telmachio, and in his pocket, a brooch in the shape of a dolphin with an inscription reading "To Rita from George." They then go to the Dolphin Club, which is owned by George Dolphin and features Rita Darling as its star attraction. Rita denies that the brooch is hers but admits to dating Forsythe before Lillian did. Determined to prove that Rita and possibly George are involved with the murders, Shayne finds Sidney Arno, the jeweler who made the brooch, and persuades him to testify in court the next day. Shayne returns to the hotel just before freelance photographer Roy Higgins, who has been following him all night, catches him. The next day in the courtroom, Rita and George again deny any knowledge of the brooch and protest that they are innocent of the murders. Working with the district attorney, to whom he had sent a letter detailing his investigation, Shayne engineers it so that Logan takes the stand. Logan admits that he has loved Lillian since childhood and was upset about her involvement with Forsythe, a disreputable lothario. Shayne then questions Arno, who confesses that upon Logan's orders, he and another jeweler not only made the brooch to throw suspicion on Rita and George, but also the knives for Telmachio to kill Randolph in case he faltered in his testimony. Logan is arrested for the murders, while Shayne is jailed for sixty days for contempt of court for having escaped from the jurors' hotel.