The entire Nelson family become embroiled in confusion by the misunderstandings caused when Ozzie invites the attractive grown-up sister of a school chum to stay with the family during a centennial celebration in the community of Hillsdale. Meanwhile, Harriet unknowingly duplicates Ozzie's hospitality by extending an invitation to a male guest. The tranquility of the Nelson home is shattered when Ozzie and Harriet are pitted against one another in a game of tolerance while trying to keep their emotions at bay. Meanwhile, Ricky is kidnapped at the Fair by a pair of hoodlums, who have come to rob the Fair office.

One morning in the town of Hillsdale, Ozzie and Harriet Nelson share breakfast with their young sons, David and Ricky. When neighbor Clara Randolph arrives with hordes of overexcited neighborhood children, waiting to attend the town's Centennial Pageant, Ozzie escapes to the relative calm of his advertising office. His boss, H. J. Bellows, who is waiting for him, announces that they have landed the account for Samuel Jones, a millionaire businessman launching a new line of ladies' girdles. With only a few days to come up with a sensational, splashy idea, Ozzie settles in to work. Later that day, he bumps into Barbara Schutzendorf, an old buddy's younger sister who has grown into a beautiful woman, and is in town to perform in the Centennial rodeo. While Ozzie invites her to stay at his house, they are watched by handsome Charlie Jones, who then tries to find out Ozzie's name. Meanwhile, at the fairgrounds, Harriet and Clara visit astrologer Clifford Hastings, who predicts that Ozzie will experience upheaval in the next few days and be attracted to younger women, and exhorts Harriet to remain understanding. Later, Ozzie's old pal Al Brown calls, and Harriet, not knowing about Barbara, invites him to stay in their guest room. On Ozzie's way home from work, he stops to talk to Clara's husband Joe, who assumes that Harriet will be furiously jealous of Barbara. Ozzie then returns home to find Harriet making up the guest bedroom for Al, and cannot bring himself to explain about Barbara. After Harriet leaves for the grocer's, Barbara arrives, and although she is standing suspiciously close to Ozzie when Harriet returns, Harriet recalls Clifford's words and says nothing. Ozzie proudly relates this to Joe the next morning, but is concerned at Joe's inference that only a wife with something to hide would not be jealous. Later, Ozzie tries to impress Harriet by buying new clothes, but she ignores them, assuming they are for Barbara's benefit. That night, Charlie shows up at the door, lying that Al Brown told him to come in his place. He is really there to see Barbara, who has recently broken up with him, and although she puts on her best dress after hearing his voice, she then pretends not to know him. At dinner, Charlie regales David and Ricky with rugged football-hero stories and flirts with Harriet, causing a jealous Barbara to turn her affections to Ozzie. While the adults dance, the boys try to unpack Charlie's padlocked bags and are thrilled to find one with an FBI identification. The next day, hoping to prove his masculinity to Harriet, Ozzie signs up for a rodeo contest in which he will have to ride a wild horse named Dynamite. On the other side of the fairgrounds, the boys eavesdrop as Charlie informs Barbara that he finally has a good job with Uncle Sam and will soon be able to ask her to marry him. Later, Jones arrives in town and is angered to discover that Ozzie has no ad ideas ready for him. Just after Ozzie calls Harriet to tell her that he must work through the night, Barbara calls with the same news, and Harriet imagines that they are spending the night together. The rodeo takes place the next morning, and after Ozzie's name is announced for the contest, Harriet runs to stop him, but turns away after an old-time rodeo star conjectures that Ozzie must be trying to impress a woman. The horse quickly throws Ozzie, who, humiliated, also realizes that he has only one hour to think up a publicity stunt for Jones. Just then, Ricky and David stumble upon two con men robbing the fair office, and while David runs to inform Ozzie and grab Charlie's bags, which he assumes are full of weapons, Ricky hides in the back of the office to watch the crooks. A bout of hiccups gives him away, however, and the crooks grab him and drive off. Ozzie arrives at the office to discover Ricky is gone, and followed by David, Harriet and the rodeo performers, jumps onto a horse to cut off the crooks on the old mill road. They arrive at the county line before the con men, but upon opening Charlie's bags, they find only girdles inside. Just then, Charlie arrives and explains that he works for Foundation Builders Inc., his uncle Sam Jones' business. Inspired, Ozzie stops the crooks' car by stringing girdles across the road, and soon journalists flood the area to photograph the ingenious trap. At the same time that Bellows and Jones show up, thrilled with Ozzie's brilliant publicity stunt, the old-timer informs Harriet that Ozzie was trying to impress his own wife, and an overjoyed Harriet finagles a big raise for her husband. As the Nelson family embraces, the rodeo performers reveal that the horse Ozzie rode to the old mill road was Dynamite, and he is declared the winner of the contest.