In 1899, Secret Service agent Tom Horn, who previously tracked down the renegade Geronimo, is assigned to locate The-Hole-in-the-Wall gang, which has been staging daring robberies from Canada to the Mexican border. During their last heist, the outlaws, led by Harve Logan, stole a shipment of treasury bank notes, which were shipped unsigned and will not be valid until signed by the president and cashier of the receiving bank. Presuming that Logan will be in need of an expert counterfeiter in order to put the bills into circulation, Tom's chief suggests that he locate Dakota Lil, an accomplished forger, and through her try to recover the money. Tom, using the name Steve Garrett, finds Lil singing in a cantina in Matamoros, Mexico, and "rescues" her and her accompanist, Vincent, who was a concert pianist before he fell victim to Lil's charms, from an "arranged" extradition. Once back across the border, Tom proposes that they head for Wind River, Wyoming, where the bank notes are thought to be, but Lil double-crosses him and leaves without him. In Wind River, Lil and Vincent go to work at Logan's gambling hall, but Tom tracks them down and she agrees that they can be partners in separating Logan from the notes. Tom takes a job as a card dealer in the saloon and discovers that another dealer, Carter, is a fellow agent and his contact man. Lil works her wiles on Logan and he eventually takes her out to the Hole-in-the-Wall gang's hideout. There he shows her the notes and asks her to forge the signatures, and she agrees for fifty percent of the proceeds. While most of the gang is away on a robbery, Tom and Carter raid the hideout and arrest one man, Dummy, so called because of the dum-dum bullets he uses which leave a victim paralyzed. On the way back to town, with Tom and Dummy handcuffed to each other, Carter is captured by the gang and Dummy is killed. Tom, having lost the key to the handcuffs, is forced to use a hacksaw to free himself, and when Dummy's body is found, the gang realizes that there is an undercover agent in their midst. Later, Lil discovers that Tom is a Secret Service agent and is about to turn him in to Logan when she sees Logan strangling Carter to death and changes her mind. Tom tells her his real name and asks for her help in capturing Logan in exchange for a lighter jail sentence. By now, Lil has fallen in love with Tom and agrees to help him. The next morning, Lil and Vincent accompany Logan to the hideout, where she sets up an engraving machine and begins to apply signatures to the notes. However, Logan has figured out that she is in cahoots with Tom and tries to strangle her, but she shoots him and rides off. Logan pursues her until Tom comes to her rescue, throwing a knife at Logan and killing him. The rest of the gang is rounded up, and Tom and Lil happily face the future together.