When druggist John Jones plans to give the opening address at the Neighborhood Druggists' Association convention held on the 4th of July at Crystal Lake, his family of three sons, two daughters, wife and mother, insists that they be allowed to come along and convinces him to buy a trailer for the outing. After various mishaps occur on the first day of travel, the Jones family stops for the night to set up camp. John instructs his skeptical sons, Jack and Roger, on the proper way to build a fire, but succeeds only in filling the trailer with smoke. His teen-aged daughter Bonnie climbs a tree to escape a playful bearcub, of whom she is terrified, and she is greatly relieved when a stranger, Tom Williams, pulls the cub away. Williams, who tells the family that he was left behind by his train, fixes the stove in the trailer and successfully intercedes when a deputy sheriff threatens to fine John for building a fire in a restricted area. At Crystal Lake, Williams courts Bonnie, and Jack falls for a vacationing girl named Mabel, who has a penchant for fast boats and peppy music, while John prepares his speech and his bookish adolescent daughter Lucy attempts to write a novel. After Jack and Mabel stave a rented motorboat, the owner, Mr. Sweeney, allows Jack to spade a large plot of land to pay for the repairs. Jack tricks Roger, an extremely entrepreneurly-minded adolescent, into digging the plot to find Indian arrowheads to sell. Roger, displeased, sneaks up on Jack kissing Mabel and takes their picture. When he threatens to show the photo to the fellows back home, Jack is forced to agree to Roger's price for the negative. Meanwhile, a Department of Justice official comes looking for Williams, really a fugitive from the Illinois State Penitentiary named Silky Walker. After Williams tricks the unsuspecting Bonnie into leaving with him in the family car, Roger finds a typed farewell note. John, with Jack and Roger, borrows Mr. Sweeney's car, which he learns too late has no brakes, and chases Williams and Bonnie. After Bonnie threatens to jump and John, unwittingly, does not let Williams get around him, Williams stops the car and concedes defeat. The family learns that the farewell note was from Lucy's romantic manuscript, and on the trip home, after John points out that they should have nothing more to do with strangers, he nevertheless stops to pick up a lone boy hitchhiking, whose large family, hiding behind bushes, then pile into the trailer.