In London, Paris, Berlin, and Madrid, four of the Paradine brothers are killed in explosions over the course of four months.
In Chicago, IL, Artmeus Gordon is backstage at his own personally produced play when a government agent, Joseph, arrives to tell him that Washington wants him back to deal with Professor Albert Paradine the Second, son of the infamous criminal mastermind. The agent points out that the audience wanted to tear Artie apart for his performance, but the ex-agent insists that it was a success. Joseph asks if he’ll reconsider if Jim comes back, but Artie points out that Jim swore that the Loveless case would be his last. However, he agrees to come back if Jim does.
In Mexico, Juanita and her three brothers are shooting at Jim, who takes refuge in a hacienda. Government agent Edward Jensen is waiting for him and warns that they have a risky mission for him. Jim immediately agrees and quickly rides out ahead of his attackers.
In Washington, Jim and Artie are reunited outside of Secret Service director Robert T. Malone’s office. Artie is less than thrilled that he has to keep his word because Jim agreed to come back, but Jim insists that he’s doing it because their country needs them. They go into Malone’s office and he explains that after a Secret Service agent retired and wrote a tell-all book, the public finds their adventures titillating. Malone admits that they don’t know what Paradine wants. He disappeared after killing his four brothers and has only recently been seen near Parsons Ridge, NV. Malone wants the agents to go there, find out what Paradine is doing, and stop him. They have had the Wanderer
refurbished and now the agents have to get in shape at their new fitness room.
After undergoing a strict training regimen, the agents head for their train with Artie still complaining. Juanita’s brothers catch up to the agents, rope them, and drag them down the street from a wagon. Once they stop, they attack Jim and Artie, and Artie convinces a couple to give him their two-seater bicycle. He rides to Jim’s rescue and they escape. As they flee, Artie realizes why Jim wanted to leave Mexico while the brothers ride after them.
The agents get to the train and head west, and Artie demands an explanation. Jim claims that he never met Juanita’s brothers and he doesn’t plan to see them again. The train pulls in at Parsons Ridge and the agents find Paradine waiting for them. He’s there to help and offers them champagne at the saloon. When Jim wonders how he knew where to meet them, Albert explains that he’s been following there every step.
At the saloon, Paradine admits that he killed his brothers because they became decent hard-working individuals. He explains that he plans to dominate the entire world by pitting the major powers against each other. Then Paradine will command the survivors after what he titles World War I. When the agents prepare to capture him, Paradine reaches into his jacket pocket and then disappears. They can still hear him, and he advises them to go back to Washington and tell Malone that they can’t stop him. The saloon girls explain that Paradine has disappeared many times before to spy on the women. An enormous explosion goes off several miles away, shaking the entire town, but the townspeople ignore it. The saloon girls explain that it’s just Paradine, playing with his bombs.
Back in Washington, the agents inform Malone about Paradine’s invisibility and point out that he can learn anything in any war council. Malone tells them to get into action but they have no idea what, and he tells them to head back to Nevada and capture Paradine.
Back on the train, the agents are reviewing their travel times when British spy Colonel Sir David Edney steps out of the back room. They recognize him from their fight against Loveless and Edney explains that he’s been riding in the engine since they left Washington. He knows about Paradine and tells the agents that England isn’t interested in a World War until 1914. Juanita and her brothers suddenly open fire on the train, riding parallel to it. Jim tells Casey the engineer to speed up and throws wood at Juanita’s brothers until the train can make its escape. Back inside the train, Jim tells Edney that it was a personal matter and threatens to throw him off when he persists. Edney goes back to the engine for the rest of the trip.
returns to Parsons Ridge and Edney’s assistant Mirabelle Merriweather is waiting for him. She tells them that Paradine is at the saloon and has invited them all to champagne. They join Paradine and Edney orders Jim and Artie to place him under arrest for the murder of Paradine’s British brother. When the agents refuse, Paradine invites Edney to place him under arrest. The agent draws his gun but Paradine turns invisible and disarms him. He reappears and tells them that he controls everything within a hundred miles, and orders them all out of the area. Another nuclear explosion goes off in the distance and Paradine tells them that they should go to his place in the desert if they need to contact him. He tells them that there are signs directing them and then turns invisible and departs. Edney says that he plans to operate separately from now on and leaves with Mirabelle.
In Washington, Secretary of State Henry Messenger summons Malone to his office and shows him the letter of protest that Edney has filed about how they’re handling the Paradine affair. Queen Victoria has contacted Messenger, and the Secretary wants answers. Malone admits that he hasn’t heard anything from Jim and Artie, and Messenger tells him to go to Nevada and handle things personally. He refuses to take no for an answer and Malone quickly leaves.
Jim and Artie follow Paradine’s hideout signs and Artie wonders what they’re going to do. He points out that they don’t have any credible charges to file against Paradine. Jim waits until he’s done and then keeps riding. They come to an oasis in the middle of the desert. Inside it is a casino tent filled with dozens of gamblers. The agents go inside and Paradine greets them. He explains that his casino attracts gamblers from all over the world. When they wonder why he wants to conquer the world when he has so much wealth, Paradine explains that it’s a family tradition. The agents talk privately and Jim suggests they arrest him. Artie points out they still don’t have any evidence to use against him. Paradine offers them some casino chips and invites them to have a good time.
Artie goes off to try his system and Jim gives him his chips. Paradine signals the wheelman to let Artie win. A bystander comes over and expresses an interest in Artie’s mathematical prowess, while Paradine has the wheelman close down the table. Jim draws him away and tells him that the wheel was rigged, while Paradine comes over and suggests that he’s won $15,000 and hints that they should consider it a bribe. He then leads them into the animal section and the agents dump the chips on his head, saying they’re not open to bribes. An angry Paradine opens a cage containing two green-skinned hulks that he explains he found in the Himalayas. Jim fights them without success and then they toss Artie aside, and then tie both agents up in a lion cage, suspended from the roof. Paradine tells them that they haven’t provided a challenge and informs them that he’s going to Washington to talk to Dr. Messenger, who is hosting a peace conference. Once the conference is over, Paradine intends to pit countries against each other in a world war. He departs for Washington, leaving the agents to a grisly fate.