When prominent New York businessman Frederick Keller is murdered, police arrest his former accountant, John Finney. Before Finney is sentenced, however, a mysterious man enters the courtroom and proclaims himself the murderer. District Attorney Cluett interviews the man, who gives his name as Joe Monday, which is an obvious alias. Joe refuses to divulge any personal information yet readily signs a confession. The press builds up the story of the anonymous murderer, and people begin writing to Joe in hopes that he is their long-lost brother, son or father. Joe's defense attorney, Steve Phillips, asks Joe to reveal his identity for the sake of those whose false hopes he is raising, but Joe maintains his silence, even when he is visited by Alice Stetson, who is searching for her brother Frank. Alice has not seen Frank since he left to fight in World War I, during which he was declared missing in France. At first she thinks that Joe is Frank and tests him by reciting poems they loved as children, but when he says that he served with Frank in the Army and saw him die, she becomes discouraged. Steve believes that this may be a clue to Joe's identity, however, and he and Alice travel to Washington, D.C. to look at the records of Frank's company. There is no "Joe Monday" listed, so they decide to question the company's two surviving members. The first, James Sawyer, is blind and cannot identify Joe, and when they try to question Paul Gilli, they discover that he has gone to New York to testify in the trial. When the trial begins, Gilli testifies that Joe is Frank Stetson and that he spied for Germany during the war. Frank was court-martialled on the basis of evidence provided by Keller and sentenced to death. Joe still refuses to testify in his defense, and the jury tries to reach a verdict. As they are deliberating, the judge calls them into the courtroom and introduces a new witness, Otto Bruckner. Bruckner takes the stand and testifies that, during the war, he was a German spy posing as a French cafe owner named Henri Picot. Keller was also a spy, and when Frank found evidence of Keller's treachery, Keller framed him. Joe finally breaks down, embraces his beloved sister and agrees to testify. He describes how Keller betrayed him and how he escaped from the firing squad but could not return to his family because he had been branded a spy. After twenty years, he found Keller and tried to force him to tell the truth. Keller pulled a gun on him, and Joe was forced to shoot him in self-defense. Believing that all evidence confirming his story had been destroyed, Joe refused to say anything about himself in order not to hurt Alice and their aged mother. Joe is acquitted after his stirring testimony, and later, a crowd cheers as he is reunited with his mother.
