Jim visits the Swanson Mortuary in Jackson Bend and has the owner prepare the body of Secret Service agent Fred Doorman for delivery to Washington. The agent then confirms that Torrey Elder is singing at the Golden Bird Saloon and visits her. She's singing for the audience and he catches her after her act, asking if Doorman mentioned the name Jim West. Torrey has a piece of sheet music, "Two Faced Stranger in the Garden," that Doorman asked her to give to Jim. She produces it and starts singing it, but one man opens fire from the balcony. When Jim goes after him, another man tries to grab the sheet music and is forced to flee.
The next day, Jim accompanies Torrey to the stage and explains that an unknown traitor killed Doorman. He has Agent Williams ride with her to Tusca just in case and then goes to the Wanderer. Jim and Jeremy go over the sheet music along with Myra Bates, a secretary from the regional office. They know that Doorman was investigating reports of a traitor in the Secret Service and left the music as a cue. Jeremy has identified the paper supplier but has been unable to find any codes or invisible ink on the paper. Jim suggests that they turn it over to Professor Montague, an expert on such matters who works at the Secret Service training academy in Denver. The agents get a telegraph on paper suppliers and one of them is the Janus Music Company in Eden, CO. They figure the title of the song related to Janus in the Garden of Eden and Jeremy leaves to visit the company while Jim rides to Denver with the sheet music.
Jim arrives at the Denver Treasury Building, which houses both the Secret Service academy and the Denver Mint. As he waits to go in, Jim notices that each visitor to the mint receives an ink stamp on their hand which glows under a red light. As Jim enters the academy, he visits his old training room and several men attack him, one with a gun. Jim disposes of them and the watching students give him a round of applause. The Academy director, Warren Blessing, suggests that they study Jim's technique and then dismisses them. Jim points out that the man with the gun was using real bullets and hit Blessing's assistant, Alan Thorpe. Thorpe only has a flesh wound, and Blessing insists that he loaded the blanks himself. Jim removes the bullet from the wall and tells him to check his bullet supplies.
Jeremy arrives at the Janus Music Company and the owner has a custom phonograph that Doorman left for anyone presenting the sheet music. As Jeremy studies it, the owner goes to answer the door. Jeremy plays the phonograph, which has a message from Doorman to Jim saying that the traitor will strike on the 18th. Before the phonograph plays the name of the traitor, thugs knocks out the owner and burst in. When they try to take the phonograph, Jeremy stuns them with a smoke bomb and heads for the door. One man grabs at him and the agent drops the phonograph, breaking it on the ground. Jeremy quickly picks it up and escapes.
The next day, Jim works with Blessing and Thorpe to review the academy files, but there's no indication that any trainee is a traitor. When they get word that Montague has returned, they go to see his lab where he's working on new agent gadgets. Montague is more interested in his designs than the code until Jim explains that it may reveal the traitor's identity. As Jim leaves for an intelligence meeting with Thorpe and Blessing, Montague suggests that he might take a look at the academy's "living room."
Jeremy boards the Wanderer for Denver and tries to reassemble the broken phonograph cylinder.
Jim, Blessing, and Thorpe return from the meeting, going past the sewer construction outside. As they go in, Thorpe explains the security procedure about the ink stamps, assuring Jim that anyone without a stamp is arrested. As they go to Blessing's office, the director's assistant Thompson comes out, glances around nervously, and tells Blessing that he left a message from Washington on his desk. The message informs Blessing that Baron Klaus Esterhazen of the Estavian Army is on an intelligence exchange tour and visiting the Treasury building Jim tells the others that he's going to see Montague on his own. When Thorpe advises against it because of the traitor, Jim admits that he's hoping to lure the traitor out.
Thorpe accompanies Jim anyway and on the way he shows the agent the "living room." The room is rigged with a variety of booby traps and the students must make their way across the room to a safe. Once they complete their assignment, Jeremy come sin, disguised as Esterhazen, and lectures the students for a few minutes. He then leaves, claiming he has an appointment in Denver.
Jim goes to the lab and a distracted Montague admits that he hasn't been able to find a code. Jim goes back to the train to meet Jeremy, who plays the intact part of the phonograph cylinder. Jeremy has confirmed there's nothing important going on the next day on the 18th. A messenger arrives with a note from Blessing asking Jim to meet with him because he has news on the traitor, and Jeremy stays behind to record himself on another phonograph disk as he plays and sings the sheet music.
When Jim goes to the academy, thugs in Blessing's office attack him. One of them escapes and runs into the living room. When Jim goes in after him, a gate slams down behind him and an unseen voice tells him that the traps are now real. Jim uses a telescoping rod to test for traps and then fires a dart and line at a nearby hat stand beneath the observation galley. The agent pulls the hat stand to him, setting off the intervening traps, and then gets to the balcony and climbs out of the room.
At the train, Jeremy plays back the cylinder at a faster speed and realizes what the code is.
When Jim finds Blessing, he insists that he didn't send Jim a note. Jeremy returns in his Esterhazen disguise and invites Jim to a drink in his appointed quarters. He plays the phonograph, revealing that Doorman's message is hidden as Morse code. The traitor is going after the Bureau of Engraving the next day but Doorman didn't know who he was. The agents realize that they can't train anyone and it's just after midnight so they have no time to bring in outside agents. Jim remembers the construction work and the agents go outside to confirm that the work ends that day.
The agents split up and a disguised Jeremy orders Blessing to let him tour the Bureau. Blessing has Thompson escort Jeremy into the Bureau after they both get hand stamps. Once Jim sees them going inside, he enters the sewers and follows a tunnel up and toward the Bureau.
Jeremy spots several unconscious man and turns, only to discover that Thompson has passed out as well. Several thugs surround Jeremy but the agents passes out and they haul him and Thompson into the engraving room.
Jim enters the Bureau basement through the sewers, knocking out the guard. He enters the engraving room and finds the intruders running off money. When Jim tries to arrest them, the traitor calls out from hiding that he is ready to shoot the unconscious Jeremy if Jim doesn't surrender. When the agent does so, the traitor steps out: Thorpe.
Thorpe tells his thugs to go back to work and congratulates Jim on his efforts. Jim has already figured that Thorpe is the traitor and that he faked being shot in the training room. The angry assistant explains that he never got promoted and was making a minimal salary, and couldn't resist the temptation of the money next door. Doorman found out about his operation and Thorpe had him killed in Jackson Bend. Thorpe has been having his men enter the building through the sewer and print their own money, and then reset their counters so no one would know there were two sets of money with the same numbers. When Thorpe boasts of his masterstroke to avoid detection, Jim tells him that he's already figured out that Thorpe put a contact knockout drug in the handstamp ink.
Jeremy tosses a noisemaker at that moment to distract Thorpe and his thugs, giving Jim the chance to jump them. The agents make short work of the crooks and Jim asks Jeremy why he didn't pass out. His partner explains that he also worked out how Thorpe administered the knockout drug and wore a skin-covered glove to avoid getting dosed. Jeremy wonders what they're going to do with the money and Jim says that it will have to be burned... and offers Jeremy a light for his cigar with a burning $100 bill.