The Higgins family prepares for a long-awaited cruise to Rio de Janeiro, but while father Joe bids farewell to his co-workers at the bank, mother Lil unwittingly sabotages their plans by telling insurance representative Wells that Joe is only forty-four, not forty-five, and is therefore ineligible to collect the annuity insurance that was to pay for the cruise. Ashamed to face their friends, Joe, Lil, daughter Betty and son Tommy decide to tag along with son Sidney and grandpa Ed Carson on their fishing trip. Grandpa is irritated to have his privacy disrupted, but the family sets out in their car and covered trailer. Ella Jones, a widow who feels an unrequited passion for Grandpa, follows them, but Grandpa siphons her gasoline and they escape. Back at Centerville, the town citizens are horrified to learn that enemy submarines have attacked the ocean liner on which they think the Higginses are traveling, and that the family is missing. While Joe's assistant, E. L. Beamish, takes advantage of Joe's disappearance by embezzling $100,000, the family is forced into hiding when Ella finds their campsite. They plead with Grandpa to lure her away, and after an encounter with a bear, Grandpa finally acquiesces. When Ella and Grandpa arrive at the Higginses' home, Joe's boss, Horace Cartwright, tells Grandpa that the family has been lost at sea. Grandpa's laughter and insistence that they are camping prompts Cartwright to bring a doctor, who puts Grandpa in an institution. Back at the campsite, Lil returns from a nearby general store, where she has paid for the groceries with a check. Furious that Lil has given a clue to their location, Joe rushes to the store to retrieve the check, but he is arrested for forgery, as the local sheriff believes Joe to be dead. While Joe stews in jail, Beamish blames him for the missing money, and Betty's boyfriend, Bill Williams, helps Grandpa escape from the institution. Grandpa goes to the jail where Joe is being held along with Lil and Sid, who tried to rescue him, and tricks the sheriff into releasing them. Later that afternoon, the sheriff finds out about the embezzlement and puts an all points bulletin out on Joe, which the family hears. As they flee, they are pursued by the sheriff, who, unknown to them, has been informed by Bill that Beamish is the real culprit and is on a train bound for Mexico. After a high-speed chase, Bill tells Joe that he is no longer under suspicion, and Joe and Bill jump onto Beamish's train. They apprehend him, and soon after, the family boards a cruise ship to Cuba, although Grandpa jumps overboard upon learning that Ella has followed him again.