Tod Shaw, the sheriff of the small California town of Suddenly, is courting Ellen Benson, a widow whose husband was killed in the Korean War. Ellen and her eight-year-old son Pidge live with her father-in-law, Pop Benson, a retired Secret Service agent. Ellen, who has become embittered by her husband’s death in battle, is overprotective of Pidge and will not allow him to see war movies or own toy guns. One Saturday morning, Ellen is dismayed to discover that Tod has bought her son a toy cap pistol, prompting her to break up with him. Later, Tod learns that a special train carrying the U.S. President will be arriving at the town’s railroad station late that afternoon. The president will de-train, then travel by car to a nearby ranch for a fishing vacation. Tod is instructed to coordinate the local security procedures and, after requesting assistance from the state police, meets with members of the advance secret service team, led by agent Dan Carney. Carney assigns his men to inspect and secure all the buildings overlooking the station, including the Benson house. Carney is delighted to learn that his former boss Pop lives in Suddenly. At that moment, Pop is trying to fix a temperamental television set, but the set explodes and Ellen has to phone the local repairman to come to fix it. Soon after, John Baron, Benny Conklin and Bart Wheeler, hired killers posing as FBI agents, arrive and ask to inspect the house. Pop proudly informs them that he was President Coolidge’s bodyguard, then inquires about why Baron and his men are there. After Baron orders that no one leave the house, he tells Pop about the president’s arrival. At the station, Carney reveals to Tod that security will be very tight because an informant told authorities about a presidential assassination plot. After Pop questions the FBI’s involvement in the president’s security, Baron tells him that they have been called in because of information about a potential assassination attempt. When Tod brings Carney to the house to see Pop, Baron shoots and kills Carney and Conklin seriously wounds Tod in the arm. Baron then threatens to harm Pidge unless they follow his instructions. After the killers set up a heavy recoil rifle on a metal table at a window overlooking the depot, they hide Tod’s patrol car in the garage. Later, Baron states that they are hired assassins and boasts that during the war he won a Silver Star for killing twenty-seven Germans. Tod assumes they will be killed when the men leave and quietly asks Pop if he has a gun in the house. After Baron sends Conklin to check on the situation in town, Jud Kelly, the television repairman, arrives and Baron takes him prisoner. While they wait, Baron admits to Tod that he has no idea who is paying him the half-million dollars to kill the president. In town, after Conklin fails to respond correctly to a deputy sheriff’s questions, Conklin draws his gun and shoots the deputy, who returns fire. Although state troopers have been instructed to take Conklin alive, Conklin is killed in a shootout. At the house, Baron orders everyone into the cellar. When one of the Secret Service agents comes looking for Carney, Baron tells Ellen that unless she can convince the agent that Carney, Tod, Pop and Pidge have gone to the ranch, he will kill them all. Satisfied by Ellen's explanation, the agent leaves. After emerging from the cellar, Tod taunts Baron about his war experience and suggests that he was court-martialed as a psychopath. Meanwhile, Pop secretly suggests to Jud that he clamp a high voltage lead from the television set to the metal table. Pop then fakes a minor heart attack and sends Pidge to get pills from the drawer in which he has hidden his gun. Pidge switches his toy gun for the real one. A few minutes later, after Pop has spilled a glass of water on the floor near the table, Wheeler steps in the water, then touches the rifle and is promptly electrocuted. His death throe spasms cause the rifle to fire several rounds. After police at the depot return fire, Baron cuts the electricity and shoots Jud, then slugs Pop. Pidge shoots at Baron, but misses. As the train approaches the depot, Baron takes aim with the rifle but is astonished when the train, having been signaled not to stop, goes straight through. Ellen, forced to reconsider her feelings about killing, picks up Pop's gun and shoots Baron. Tod then shoots, ending the killer’s life. Later, as Pop recovers, Tod emerges from the hospital and tells Ellen that although Jud died, his deputy will live. Ellen then asks Tod if they can resume their courtship.