On the trail of Jim and Artie, Malone follows the signs to Paradine's headquarters and casino. The villain is there to greet him, well aware of who he is, and tells Malone that he won't tell anyone else that he's the director of the Secret Service director. Malone is surprised at the warm welcome but Paradine tells him that he's impressed and invites him in to freshen up. They have champagne and Paradine claims that Jim and Artie went on a fishing trip after they assured themselves that Paradine hadn't killed his four brothers. He invites Malone to join the agents, but Malone hates fishing so Paradine has Yvonne and Daphne wait on him.
Jim and Artie, suspended over the lion's cage, swing free to pull themselves clear. However, Paradine and his hulks are waiting and he tells them choose between a basket or two curtained passages. Jim chooses the basket and Paradine happily tells him that he's chosen a slow death rather than a quick and painless ones. Artie slips away while the hulks escort Jim to the Snake Pit Lounge. The agent is tied up and placed in front of a wicker basket, and an elderly snake charmer comes out to have a snake attack Jim. The snake charmer is actually Artie in disguise, and he manages to lull the cobra asleep. He then offers to turn Jim's ropes into cobras and invites the hulks to help. Artie ties them up with Jim's ropes as part of the act and then he and Jim run for it. As they steal two of Paradine's zebras to escape, Artie explains that the real snake charmer is an old performing friend.
As the agents ride cross-country, Juanita and her brothers fire at them. Jim finally admits to Artie that he left Mexico to avoid them and then circles around while Artie draws their fire. He grabs Juanita, who explains that she's mad at him because she turned down the governor's proposal of marriage to be with him. Juanita apologizes for her temper and calls her family off, and the couple kiss.
The agents return to Parsons Ridge and prepare to take the Wanderer back to Washington. They have express access down the rail line and figure they can beat Paradine there. Malone arrives and he tells them that Secretary of State Messenger sent him to stop Paradine, but he doesn't know why. The agents tell them about Paradine's plan to disrupt the secret peace conference, and Malone tells them that he ran into Edney, who is heading to Washington to file another complaint against Jim and Artie. As the three men board the train, they notice a hot-air balloon overhead... unaware that it's Paradine's private transport.
The agents arrive in Washington and meet with Messenger, who admits that someone that could be an invisible man has been "seen" around the conference. Messenger insists that the agents prevent Paradine from disrupting the peace conference, the crowning moment of his career. Artie tells him that he plans to infiltrate the conference disguised as an ambassador and Messenger tells them to leave.
At their hotel room, Artie disguises himself as the Lichtenstein ambassador and Jim forges his papers. However, when Artie enters the conference building, Paradine is already there wearing the same disguise. He says hello to Artie and then slips away using his invisibility actuator. Artie meets Jim back at the hotel and tells him what happened, and they realize that the invisible Paradine spied on them and learned about Artie's plan. Jim has two tickets to the conference's observation gallery and the agents leave for the conference hall.
As Messenger calls the conference to order, the agents try to spot Paradine. The ambassadors immediately begin arguing over a German invasion of Cuba. The Russian, German, and Spanish ambassadors argue back and forth and a fight soon breaks out. The agents realize that the ambassadors have started fighting without Paradine's help and leave.
When the agents return to their hotel room, Paradine is waiting for them and complains that the war isn't breaking out the way he planned, wrecking his master plan. He finally boasts that he has planted bombs in the major capitals of the world and plan to detonate one in ten hours to start the war. Paradine dares the agents to find them and then slips out invisibly when the agents try to capture him.
The agents have no idea where to start and begin checking the streets. They spot Yvonne and Daphne going into a hotel and approach them for champagne. The sisters admits that they know where the bomb is hidden but refuse to talk. Jim grabs Daphne and drags her into the next room, threatening to do whatever he has to. He then tries to seduce her and although she refuses to talk, they end up in bed. When Jim emerges later, he finds Artie holding a sleeping Yvonne. Jim has confirmed from Daphne that Paradine hid the bomb in the State Department building and plans to set it off at 9 a.m. Messenger has called all of the ambassadors together and Paradine has set the bomb to go off at that moment.
Jim and Artie meet with Messenger but he refuses to call off the meeting despite the risk. He tells the agents to find the bomb and Jim goes back to the hotel to try to get Daphne to reveal the bomb's hiding place. He promises that he won't forget her and she tells him that the bomb is hidden beneath the conference floor. As Jim goes, he realizes that Yvonne has taken her sister's place and vice versa.
Paradine goes to the State Department, invisible and riding in a carriage. Once he gets inside, he has the hulks capture Jim and Artie, who realize that the sisters set them up. Paradine then goes into the conference room and invisibly replaces one ambassador's case with his own.
The hulks tie Jim and Artie up in the boiler room and stand guard and the steam pressure starts to go up. Jim manages to open a steam valve, blinding the hulks, and then cuts himself free with his boot knife. He frees Artie and they head upstairs, but Daphne and Yvonne are waiting. They explain that Paradine forced the sisters to betray them and want to help, and tell the agents that the bomb is in the conference room.
As Messenger addresses the ambassadors, Jim and Artie run in and announce that there's a bomb. When the ambassadors panic, Messenger orders them all to stand still and check their cases. They find nothing, but the agents realize that Paradine has substitute the bomb for Messenger's case. Paradine shuts down his invisibility actuator and tells everyone that he is invulnerable when he's invisible, so he is there for a front-row seat at the explosion. However, when he turns on the actuator, he discovers that it is broken. While Paradine panics, Messenger tells Artie to defuse the bomb and the agent does it just in time.
Edney emerges and handcuffs Paradine to himself and Jim warns the agent that he's out of his jurisdiction. The spy concedes the point and turns Paradine over to them before leaving for England, handcuffing the mad scientist to Messenger. Messenger wants to discuss Paradine's invisibility device and assures him that there'll be another chance for mass destruction. As Artie examines the actuator, Jim admits that they may be getting too old to fight evil.
As Messenger and Paradine leave the building, they turn invisible. Inside, Artie tells his partner that he got the actuator working but Jim throws it into the Potomac. The agents find Daphne and Yvonne and decides to spend the next week with them. The invisible Messenger and Paradine, still handcuffed together, run up and Jim tells them the actuator is in the river. They dive in to find it, arguing among themselves, while Jim and Artie depart in Paradine's hot-air balloon.
Washington - 1980
Robert Malone the 4th tells the audience that they have found the actuator in the old records, but dismisses it as an absurd contraption that couldn't possibly works. He then turns it on and disappears.