In 1892, at Oxford University's Commemoration Day, roommates Jack Chesney and Charley Wykeham invite their girl friends, Amy Spettigue and Kitty Verdun, to their rooms for lunch, as Charley's rich aunt from Brazil, Doña Lucia d'Alvadorez, is expected to arrive by train and can act as chaperone. However, Doña Lucia, whom Charley has never seen, has not arrived when the girls show up, so they promise to return later. Jack's father, Sir Francis, arrives to tell Jack bad news: upon inheriting his title, Sir Francis also inherited debt, and now the Chesneys are noble, but poor. Before Sir Francis leaves, Jack suggests that he consider wooing Charley's aunt, who is a widow and millionaire. Then Charley gets a telegram saying Doña Lucia has been delayed, just as the servant Brassett announces that the girls have returned. As this will be their last opportunity to be together before the girls's guardian, Mr. Stephen Spettigue, takes them to Scotland, Jack convinces Charley to impersonate his aunt, so they will stay. Wearing a black bombazine dress from an upcoming amateur theatrical in which he will play an old woman, Charley is introduced as his "aunt" to the girls, and then to Spettigue and Sir Francis, who show up later. The group leaves to dine at the local pub, but Charley abruptly leaves them, disgusted by Spettigue's brash attempts to woo "Doña Lucia" for her money and anxious to visit with Amy as himself. After Sir Francis intimates to Jack that he is considering proposing to the "aunt," Jack proposes to Kitty when they are alone on the campus grounds. Meanwhile, Charley has changed clothes and proposes to Amy. However, their expressions of affection are interrupted by Spettigue, who was offended by Charley's brush-off at lunch and wants to take the girls away. To delay Spettigue, Charley tells him that his "aunt" is really attracted to him, just shy. Then, after sneaking away, Charley returns in the bombazine dress. With a flirtatious chase over the campus grounds, he eludes Spettigue, then refuses a marriage proposal from Sir Francis, before rushing off again. While Sir Francis rejoices that the unattractive "doña" has rejected him, an attractive woman enters the campus courtyard looking for Charley. When Sir Francis realizes that the woman is Lucy, an old flame whom he has not seen for twenty years, he tells her about Charley and his "aunt." Meanwhile, the girls ask Charley's "aunt" to convince Spettigue to write a letter giving consent for them to marry their beaus, although Amy fears that Charley's frequent disappearances indicate that he does not love her. Hampered by his disguise, Charley does his best to convince Amy that it is not true. Then the happily reunited Sir Francis and Lucy, who identifies herself as a widow, Mrs. Beverly Smythe, join the group and Lucy claims she knew Doña Lucia's husband intimately. During tea, urged by Lucy, Charley regales them with stories he makes up about life in Brazil. That evening at the Red Rose Cotillion, dressed as his "aunt," Charley convinces Spettigue to give his written consent for the girls to marry by accepting his proposal of marriage. After a quick change of clothes, Charley manages a dance with Amy, but then changes back to the bombazine in time for Spettigue to announce the engagements of his wards and himself. However, Charley's dress rips, revealing his identity and the betrayed Spettigue claims that the letter is invalid, as it is addressed to Doña Lucia. Lucy, who is attending the ball with Sir Francis, interrupts and takes possession of the letter, explaining that she is the doña, Charley's real aunt, and Spettigue leaves in a huff. Because of the letter, the two young couples are able to marry. Lucy forgives Charley for impersonating her and the cotillion continues.
