Jim escorts U.S. Archive curator Silas Grigsby into the Panhandle Strip and the nearest border town. A number of thugs are waiting for them, and Artie is also present, disguised as a squint-eyed gunfighter. Artie tells them to dismount and then secretly warns Jim that the man to watch out for is Clint Cartwheel. As Jim and Grigsby discount, a man throwing knives, Pima, takes offense at Grigsby and tells him to bow and apologize. Jim refuses and attacks the man, but Cartwheel steps in and knocks him down. As the henchman prepares to finish off Jim, Carlotta Waters rides up and orders him to stop, and then welcomes Jim to the Panhandle. When Pima puts up a fight, Jim knocks him out, and then apologizes to Carlotta for being late.
Carlotta and Cartwheel escort Jim and Grigsby to the main house. After searching the two men, they blind fold them and take them inside. Jim keeps tracks of the number of footsteps and turns, until he's put in an elevator. It descends below ground and Jim is spun around before being taken down one of four corridors. Carlotta then opens a vault, revealing a hallway with an acid pit and a narrow bridge reaching across.
After crossing the bridge, Jim and Grigsby are brought into a darkened room. Their host, Victor Freemantle, appears in a spotlight and welcomes them, and then tells Grigsby to inspect the item he stole: the U.S. Constitution. Grigsby confirms that it's authentic and Freemantle demands $1 million in gold bullion and sovereignty of the Panhandle Strip. He tells Jim to deliver his demands to Jim and has Cartwheel knock him out, while he keeps Grigsby as a hostage.
Outside, Cartwheel prepares to whip Jim before sending him on his way. Artie intervenes, claiming that Jim sent him to prison for five years, and pays Cartwheel $50 to drag him behind his horse for a while. Cartwheel agrees and Jim "accidentally" breaks free while Artie tosses an exploding bottle behind them to cover their escape.
The agents return to the Wanderer and confirm that Freemantle is a revolutionary who first tried to establish an empire of his own in South America, and now plans to annex Texas and Baja California. They have a scale model of the fortress from the Mexican Museum and are able to track Jim's movements until he was taken into the old mine shafts below the fortress. The agents have no choice but to check all four hallways.
To infiltrate the fortress, Artie rides into town posing as a traveling alcohol salesman offering all-you-can-drink samples, while Jim hides beneath the wagon. The saloon owner, Brad Logan, doesn't want the competition and tells Pima to set the wagon on fire. Pima almost sets Jim on fire but Artie douses the torch and Pima with seltzer water. When Artie explains that he's only there to sell samples and offers Logan cheap prices, the saloon owner agrees to try a sample. The first bottle emits smoke, letting Jim slip away in the confusion. Artie offers Logan a second bottle and he draws a gun and forces Artie to open it first.
While Jim gets into the fortress using a mechanized grappling hook and winch, Artie opens the bottle, firing a spring snake. He ducks into a wicker basket on the back of his wagon, and Logan and men discover that Artie has seemingly vanished inside. Meanwhile, the agent has gotten to the fortress gate and Jim lets him in, and they attach putty to the gate hinges before splitting up to search for the elevator.
Jim makes his way in through Carlotta's room and manages to avoid her seeing him. However, when he's forced to retreat back into the room to evade the guards, Carlotta captures him and prepares to shoot. Freemantle knocks on the door and Jim disarms the distracted Carlotta and forces her to say that she's dressing. Freemantle says that there's been a disturbance and orders her to stay in her room, and contemplates going in only to find the door is locked.
Once Freemantle leaves, Carlotta flirts with Jim and offers to betray Freemantle. She figures her employer's cause is doomed and offers to deliver Grigsby to Jim in return for a considerable sum. Jim agrees and she tells him which hallway to take in the basement. They share a kiss and Jim leaves, and Carlotta goes out into the hallway to sound the alarm. Anticipating her betrayal, Jim and Artie are there and knock her out with chloroform, then take her back to her room and sprinkler her with alcohol to make it appear that she passed out while overdrinking.
The agents go to the basement and find the elevator hidden behind a fake wall. They discover that it's rigged to an alarm and use thermite to cut through the floor and lower themselves down.
Freemantle goes to see Carlotta, smells the alcohol, and figure that she's passed out. He leaves, unaware that she's starting to wake up.
Jim and Artie follow Carlotta's directions, blast through the vault door and discover that the bridge has been removed. They fire a grappling line across and Jim uses a arm-mounted pulley to traverse over the acid. He then grabs the Constitution and heads back.
Cartwheel finds Artie's disguise in the whiskey wagon and Freemantle realizes that the agents are going to get the Constitution. They take a squad of men to the basement, arriving just as Artie pulls Jim back across the acid. Freemantle hears Artie confirm that the Constitution is a fake and explains that the whole mineshaft setup is to lure any thieves in. When Artie figures that the real Constitution is hidden somewhere in the fortress, Freemantle admits that it's hidden "where lady fair can stare at it for hours." The agents start a fight and defeat Cartwheel and the others. When Freemantle charges at Jim, he slips and falls into the acid, meeting a grisly demise.
Realizing that Freemantle is referring to Carlotta, the agents go back to her room and finally locate the Constitution hid inside of her mirror. As Freemantle's men close in, Artie activates a smoke emitter on his back and they run outside, covering their retreat with blinding smoke. Jim shoots the putty on the gate, blasting it off its hinges, and they make their escape.
The agents go to the Wanderer and Jim prepares to go back to rescue Grigsby. However, Carlotta arrives with Grigsby, just as she promised. Grigsby asks to see the Constitution but immediately declares the copy the agents have to be a fake. Jim points out that it's the same one that Grigsby confirmed as real earlier, and Grigsby and Carlotta draw guns. They know the agents took the real Constitution from the fortress and demand that the agents hand it over. Jim says it's in a secret compartment and offers to open it, but Carlotta figures that it's a trap and has Grigsby get it. The agents warn that Grigsby will be dead before he hits the floor, but the curator ignores their warning, opens the cabinet, and is shot by a pair of mounted pistols. Carlotta, distracted, is easy prey for Jim and he disarms her.
Later, Artie is packing the model away to return to the museum when the agents receive a message by carrier pigeon telling them that the U.S. has turned the Panhandle over to Mexico to deal with it. Jim decides to write Carlotta a letter in prison, much to Artie's dismay.