In Mexico City, a little girl admires paintings in the park and Kelly chats with her briefly. Scotty is looking over paintings for his mother and the artist says that it shows an atomic explosion and is dedicated to peace. Kelly spots one of his old Hollywood buddies, movie star Sean Christy, and goes over to greet him. Sean punches him and walks away, and the little girl offers Kelly her flower.
Later, Kelly and Scotty report to the embassy and turn in their expense accounts to the accountant, Shelby Clavel. Kelly is over budget again and he points out that the dinner cost is for their meal together. He agrees to pay it and asks Shelby to get him Sean's address. She tells him that Sean can't be in Mexico because he'd have a tourist card and it would have gone through her. Shelby explains that there has been a rash of mistaken identities recently and they received reports that novelist Tom Keith was in town. They day before that it was Jim Stanley, the middleweight champion. None of them had tourist cards. Kelly is sure that he saw Sean but Shelby doesn't believe him.
As the agents leave, two men accost them with guns and direct them into a town. They're taken to a penthouse and knock out Kelly, telling Scotty they only need one of them. They then ask Scotty what he knows about Sean and the agents says he doesn't know anything other then that he's a movie star. One of the thugs mutters in Spanish and Scotty translates. He asks why Senor Munoz wants them dead and keeps their attention long enough for Kelly to jump them from behind. However, Sean comes out with a gun and tells them to surrender. He tells Kelly that he has a problem and that Munoz is a great man. The actor says that a great uprising is coming. One of the thugs cautions him not to say anything else, and Sean tells Kelly that he wasn't supposed to see him. The actor says he has instructions to kill him if it's a problem, and refuses to cut them in. Sean tells them to go home and that they need to leave by the next day.
That night, Kelly takes Shelby out for drinks and she says that it's business. She's considered his meeting with Sean and Kelly is disappointed that she hasn't fallen for his charms. Agent Hubbard comes over and identifies himself as a Mexican agent, and Shelby goes to meet her date. Hubbard escorts Kelly outside and explains that he's been looking for Sean and the other celebrities for the last week. Kelly assures Hubbard that he knows Sean passingly and explains that the actor always seemed like a swashbuckler, fighting for causes. Hubbard says that Sean has been sounding off about a small but proud country that needs protecting, and the U.S. doesn't want anyone organizing a revolution within Mexico. The government wants them out and Hubbard tells Kelly it's his job to get them out. When Kelly mentions Munoz, Hubbard admits that he's heard of it but hasn't put a fix on it yet. Kelly knows that Sean has had him followed and plans to follow them back to Sean.
The next day, Kelly and Scotty catch a taxi and the two thugs follow them. The agents get out and take a rowboat out into the river, and the thugs rent another boat and go after them. Kelly and Scotty put to shore and go to the nearby park to buy some balloons and take a walk. The thugs watch the balloons instead of the agents and discover that the agents have tied the balloons to a dog. They notice that someone has released all of the balloons and go back, and miss the agents eating with a family at an open-air restaurant.
The agents follow the two thugs but lose them when villagers block the road with their herd. They go to a local bar and stage a loud drunken conversation about how governments can't be trusted. A man, Ramone Vasconcelos, comes over and introduces himself as a local newspaperman. He recognizes Kelly the tennis player and asks to listen to Kelly's complaints. Kelly continues to talk loudly against the U.S. government and he's glad to have Vasconcelos take down his statements for publication. Two American customers take offense, come over, and start a fight. The agents quickly knock them out and go back to speaking against the U.S.
Kelly's statements are soon published and he is soon taking angry calls from tournament organizers. He finally gets a call from Sean and says that he said everything in the article. Sean wants a meeting and Kelly agrees to talk to him somewhere in public and alone. The agents go to the pyramids outside of the city and find Sean waiting for them at the top. Kelly and Scotty eventually make it up and Sean tells him that the Mexicans have developed faster since their revolution. He asks Kelly if he was serious and Kelly says that he was, but Sean points out that he's a spoiled star. Scotty says that Sean isn't any different, and Sean calls over Tom Keith the novelist. When Scotty points out that he hasn't been writing anything in five years, Tom says that he's finally become an activist and is there to hear them talk. Sean tells them to express their beliefs, and the agents say that they believe in right and wrong and doing something about it.
The two activists take the agents to the manor where the other celebrities have assembled. Their host, Rafael Munoz, introduces himself and explains that everyone he has gathered can create international headlines. Munoz wants Kelly to make headlines and tells the agents that everyone present has been trained to invade the beach of a neighboring country where the government is exploiting the lower classes. The local rebellion will meet them, take the radio station, and announce their liberation. However, Munoz warns them that they will have to prove their political convictions and Scotty immediately agrees.
Munoz continues, saying that the support of so many celebrities will show that their cause is just. However, he says that they'll need more proof than words and his thugs bring in Sean. Munoz tells the agents that Sean is the traitor because he broke cover and stopped the thugs from killing Scotty and Kelly. He gives Kelly a gun and warns him that he's on trial. If he refuses, it will prove he's not capable of joining their cause and Kelly will suffer the consequences. Scotty tells Kelly to kill him, asking him to believe in him. Kelly takes the gun and fires at Sean point-blank. Sean is wearing a bulletproof vest beneath his jacket and Munoz congratulates Kelly on passing his test.
Sean leads them away, and Scotty tells Kelly that he felt the vest when he briefly touched his shoulder. They go to the armory and put on uniforms. Scotty pulls the pin on a grenade and drops it, and Sean and Kelly dive for cover. As Tom comes in, Scotty reveals that the grenade is filled with sand. All of the grenades are the same and they confirm that the guns have no firing pins. Scotty tells them that Munoz plans to sacrifice them as martyrs in his invasion scheme and get major headlines for his cause after warning the enemy in advance.
Munoz comes in with his armed men and congratulates them on figuring out his scheme. When Scotty wonders how he plans to explain their deaths to the others, Munoz says that he'll say that they betrayed the cause and then he'll dump their bodies on the beach and make them martyrs. He invites them to pick a wall to be shot against and Kelly distracts them while Scotty throws sand from the grenade into a thug's face. A fight breaks out and Tom is shot, and Scotty, Kelly, and Sean make short work of Munoz and his men. Sean and Tom go up to tell the others that they've been played.
Later, the agents go back to the artist in the park and spot famous movie star Sheldon Leonard walking with a woman. Kelly expresses his admiration and compliments Sheldon on his attractive wife. He takes offense and punches Kelly, saying the much younger woman is his secretary, and the little girl offers Kelly another flower.