Wealthy Samuel Fulton is getting older and he wants to test his family before leaving his money to them. He anonymously grants them $100,000. Harriet Blaisdell moves the family into a mansion and makes plans to marry her daughter Millicent off to a socialite rather than her soda jerk boyfriend Dan.

In a Tarrytown, New York mansion in the late 1920s, the richest man in the world, skinflint hypochondriac Samuel G. Fulton, plans to leave his fortune to the children of the love of his life, the late Millicent Blaisdell. He explains to his lawyer, Edward Norton, that it was only because Millicent spurned him for a bookkeeper that he turned to business and made his millions, but that he always wished he could have had a family with her instead. When Norton suggests that the Blaisdell family, consisting of father Charles, mother Harriet, and children Millie, Howard and Roberta, might spend his money foolishly, Sam decides to visit them at their home in Hilverton, Vermont. Bearing a newspaper advertisement asking for a border, Sam introduces himself as a painter named John Smith. A bewildered Harriet, who does not realize that Sam himself has placed this ad, allows him to stay for one night in Grandma Millicent's old attic room. He soon finds himself partaking in the kind of home-cooked meals and cheap cigars his doctors had assumed would cause him certain death. Roberta immediately likes Sam and, after urging him to paint for her, declares she also appreciates the paint-splattered canvases which he calls "modern art." The middle-class Blaisdells are a happy family, except for Harriet, who despairs that Millie will befall the same fate as her grandmother by spurning the attentions of rich rake Carl Pennock for those of soda jerk Dan Stebbins. As Harriet attempts to push Millie into Carl's arms, Sam convinces Charles to let him stay on as a border by offering to work part-time in the store alongside Dan. One night, Millie and Dan burst in and announce their engagement. While everyone except Harriet celebrates, Norton arrives and reveals that he represents an anonymous benefactor who has bequeathed the family one hundred thousand dollars. Harriet immediately plans how to spend the money, and declares that now Dan will never be able to support Millie "in the manner to which she will become accustomed," causing Dan to storm out angrily and Millie to burst into tears. Harriet then states that the family mongrel must be replaced by French poodles and that Sam must move, causing Roberta to run out crying. Within days, Harriet buys the biggest house in town, and the Blaisdells are the toast of Hilverton society, all of whom believe that the family has inherited millions. Millie is forced to date Carl again, although he continues to be a drunken cad. When he brings her to a speakeasy one night, Sam learns of an impending raid, and rushes there to rescue her. As they escape out a window, Carl pushes Sam back into the speakeasy and Sam is arrested. Millie and Dan both arrive to bail him out, and although Sam hopes for a reconciliation, they turn away from each other. Sam continues to work at the store under its new owner, who is even cheaper than Sam. As he works one day, he overhears Howard beg a local gambler for more time to repay his two thousand dollar debt. Posing as a novice gambler, Sam visits the card tables that night and wins back Howard's IOU, but is captured in another police raid. Dan bails him out again, then determines to leave Hilverton to find his fortune, even though Sam warns him not to let Millie go because money cannot buy happiness. While Sam visits Millie that night, Howard mentions that his IOU was mysteriously returned to him, and takes her to see Dan at the movies. There, Dan spurns Millie's attempts to talk, and when she sobs that she wishes they had never gotten the money, the Pennocks see Sam comforting Millie and assume the worst. They race to the Blaisdells' cocktail party with the news that Sam is a molester, and although the pair denies the charge, Harriet insists that Millie announce her engagement to Carl immediately. At the engagement party, Charles learns that his investments have failed and calls Norton for a loan. Sam, realizing the family cannot make wise financial decisions, instructs Norton to refuse, and Charles is forced to turn to Pennock. As soon as Pennock discovers that the Blaisdells are broke, however, he leaves with his wife and Carl. Charles informs Harriet that their only hope now is to sell the house and buy back the store, and although she faints, the rest of the family rejoices. Soon, Dan and Millie are engaged and Carl has his store back. Upon learning that he has won first prize in the local art show because Roberta has seceretly entered his paintings, Sam, still protecting his real identity, races away to avoid the press. He bids goodbye to the family, who now think of him as the Blaisdell grandfather he always wished he could be.