In the year 1901, the President of the United States, William McKinley, faces immense pressure from multiple sources, including the US Bank Examiner Henry Maxwell, to take action against a notorious gang of bank robbers who have managed to evade capture. Recognizing the threat, McKinley secretly authorizes U.S. Navy Lieutenant Richard L. Perry, played by Robert Taylor, to go undercover and gather intelligence on the criminals. He adopts the alias Joe Patrick and dives into the criminal underworld, working discreetly to uncover the mastermind behind the daring robberies, without informing the Secret Service or anyone else involved.
While infiltrating the gang, Joe-Perry catches the attention of Lil Duryea, portrayed by Barbara Stanwyck, a talented singer performing at Batiste's casino in Saint Paul, Minnesota. Lil’s stepbrother, Batiste Duryea, an imposing figure played by Brian Donlevy, owns the casino and is one of the key leaders of the criminal organization. Joe develops feelings for Lil, and she, in turn, shows an interest in him, though their budding romance is complicated by Batiste's suspicion. Jock Ramsay, a formidable enforcer played by Victor McLaglen, considers Lil as his personal girl and becomes wary of Joe’s intentions.
Despite Batiste's initial suspicion and his invitation for Joe to join the gang, Joe remains focused on his mission. Lil, sensing danger, later tries to persuade Joe to escape and run away together. Joe, caught between his duty and his feelings, agrees to leave but then has a change of heart when he learns the true depth of the gang's operations and the identity of the criminal mastermind. Before he can act, Batiste is killed and Jock is wounded during a confrontation with the police.
While incarcerated, Joe works to gain Jock Ramsay’s trust and eventually learns that the true ringleader behind the gang is the Bank Examiner, who is working in secret to sabotage the authorities' efforts. With this crucial information, Joe hopes that unveiling the boss's identity will bring justice. However, tragedy strikes as President William McKinley is shot before the letter with the critical evidence can reach him, leading to widespread disbelief and the wrongful sentencing of Joe and Jock to death.
When Lil visits him in prison, Joe confesses everything and pleads with her to seek help from Admiral George Dewey, played by John Carradine, hoping someone in authority will verify his claims. Lil, overwhelmed by guilt for her part in the events and for her stepbrother's death, initially refuses but later rushes to see Dewey, determined to clear Joe’s name. Together, they then petition the new President, Theodore Roosevelt, portrayed by Sidney Blackmer. Roosevelt is initially skeptical but becomes convinced when it’s revealed that McKinley's assassination involved a secret instruction in a letter bearing a special symbol, which only trusted officials could interpret.
In a tense moment just before the executions, an official finally confirms the true story, leading to a reversal of judgment. Joe and Lil are ultimately reunited, their faith in each other and the pursuit of justice prevailing despite the tragic events. This story blends elements of espionage, political intrigue, and romance, set against the backdrop of early 20th-century America, illustrating the dangerous world of criminal underworlds and the courage of those who seek to bring crime to justice at great personal risk.
