Jack Chesney and Charley Wykeham, two undergraduates at Oxford University, are in love--Jack with Kitty Verdun, ward of Stephen Spettigue the Oxford solicitor, and Charley with Amy Spettigue, the solicitor’s niece. However, neither knows quite how to express his love and his (hopeful) plans for the future to his girl—until Jack comes up with a plan: It just so happens that Charley’s aunt, a wealthy widow from Brazil (“where the nuts come from”) is visiting that very day. So, why not have a luncheon for the girls and Charley’s aunt? Surely during the afternoon each boy could sneak a few moments alone with his girl to express his sentiments. Charley has reservations, but finally consents (partly because he knows old Spettigue is out of town for the day); and the invitations are sent to the girls via Brassett, Jack’s college scout.
Then, to top off the plans, the two students decide to invite Lord Fancourt Babberley (“Babbs”) to entertain the aunt, while they entertain their girls. Babbs, however, is not entirely keen on the idea but sympathizes with his two friends because he, too, is in love (“I’m always wanting to be alone, and hear the birds sing”). He met his girlfriend on a cruise and has since lost track of her—but hopes to someday find her and rekindle the romance.
Then one more is added to the luncheon when Jack’s father, Colonel Sir Francis Chesney arrives and announces his finances are in disarray and he and Jack will have to lower their style of living. In an effort to alleviate the situation, Jack invites him to the luncheon to woo Charley’s aunt and hopefully marry into her millions.
But all plans go awry when the aunt, Donna Lucia d’ Alvadorez, telegraphs that she won’t be arriving for a few more days. Without the aunt as chaperone, Charley and Jack quickly realize the girls won’t come to lunch. The answer presents itself, however, when Babbs enters wearing women’s clothing he is trying on for a theatrical review. Jack immediately seizes him and calls him Charley’s aunt, the girls arrive at the door and are let in, and Babbs, not at all used to this new role, is suddenly forced to masquerade as the doting aunt from Brazil.
All three boys, especially Babbs, are uncomfortable; but things, of course, only get worse with the arrival of Jack’s father and then (unexpectedly) of Spettigue. Both men are charmed by “Donna Lucia” and begin to vie for the rich aunt’s hand as Babbs leads them both on a wild chase. Then, the high jinx kick into high gear when the real Aunt Donna Lucia appears, recognizes what is going on, and plays along by taking a different name. Adding the last wrinkle to this hilarious plot is her traveling companion: Ela, Babbs’s long-lost love.
Comic confusion reigns supreme, but all is finally sorted out in the end when, Babbs, still posing as the aunt, tricks Spettigue into agreeing to the marriage of Kitty, to Jack; the real aunt marries Jack’s father; Charley gets Amy; and Babbs regains the girl he loves.