While on a train to Wainwright College, incoming freshman Andy Hardy vows to himself that he will not allow anything to interfere with his academic endeavors. No sooner does the girl-crazy Andy say this, however, than he meets two attractive young women. Andy is pleasantly surprised to learn that Wainwright is now admitting women, and that one of the two co-eds, the sophisticated Kay Wilson, will be his classmate. The other woman, Lee Walker, an uncontrollable flirt, is traveling surreptiously with her conservative twin sister Lyn, who is also enrolled at Wainwright. With help from their sympathetic aunt, the sisters have tricked their father, who wants the twins to be separated so that they can develop their own identities, into believing that Lee is in Vermont, but now realize they only have enough money for food and lodging for Lyn. Lee quickly overcomes the problem by convincing Andy, who is unaware that she is a twin, that upperclassmen will humiliate him and steal his money if he is found with too much cash. After Andy gives Lee ten dollars for safekeeping, his father, Judge James K. Hardy, wires him some more money on the train, and Andy frantically passes it on to Lee. Andy then has to deal with his mounting jealousy of fellow passenger Dr. M. J. Standish, whose mature, eloquent ways are making a positive impression on Kay. Although Dr. Standish assures him that his interest in Kay is strictly friendly, Andy becomes upset when he learns that Dr. Standish is driving Kay onto campus. Later, at Wainwright, Andy, whose father is an influential alumnus of the school, is sent to see his faculty advisor, a dean, and is shocked to discover he is Dr. Standish. While talking with the dean, Andy realizes he was hoodwinked by Lee, but assures Dr. Standish he will resolve the problem on his own. Andy then tracks Lee to a boardinghouse and demands his money back. Lee, who is still posing as Lyn, tearfully promises she will pay him back with money earned from her new singing job at a club called Joe's Place. Soon after, however, Lyn receives a concerned, cryptic telegram from her father and starts to cry in front of Andy. The still unsuspecting Andy comforts Lyn with a kiss, a gesture witnessed by Kay, and makes a date with her for that night. Andy then convinces the jealous Kay that he is serious about school and eagerly suggests they become study partners, beginning that night. Now wanting to break his date with Lyn, Andy rushes to the off-limits Joe's Place, where Lyn is rehearsing, and is startled to find both twins there. Unable to hide anymore, Lyn and Lee reveal their situation to Andy and tell him their suspicions that their father knows Lee is there and is calling that night to confront them. To avoid their father's wrath, Lee intends to take a train for Vermont, while Lyn fills in for her at Joe's Place. Andy endorses their scheme, but then hears from Dr. Standish that Mr. Walker called him earlier that day to talk about Lyn. Sure that Dr. Standish knows about the twins, Andy goes to their boardinghouse that night to telephone their father, but has to wait for his return call. Kay, meanwhile, surmises she has been stood up by Andy and gladly accompanies Dr. Standish on a tour of the campus. When they reach a romantic spot, Kay gives Dr. Standish an impulsive kiss, then apologizes and runs off. Back at the boardinghouse, Andy finally gets Mr. Walker on the phone. After determining that Mr. Walker is unaware of his daughters' subterfuge, Andy, posing as Dr. Standish, convinces him that the depressed Lyn needs to be temporarily reunited with her twin. Having solved the twins's immediate dilemma, Andy tries to sneak out of the women-only boardinghouse, but is caught by the landlady, Mrs. Gordon, who informs him that she will have to report him to Dr. Standish. Before class the next morning, Dr. Standish asks to see Andy later, and fearing the worst, Andy prepares to leave school. On his way out, however, Andy runs into his father, who has just recuperated from a tonsilectomy, and pretends all is well. Dr. Standish then joins the two, and begins to reminsce about his carefree days as a student at Wainwright, when Judge Hardy was his moral mentor. The judge and the dean confess that they had arranged to test Andy's character by giving him a "responsibility" in the form of a "lollapalooza" named Lyn. At that moment, a grateful Lee and Lyn show up to pay Andy back and surprise Dr. Standish, who never suspected that Lyn had a twin. Dr. Standish then assures Andy that all is forgiven. Later, Dr. Standish admits to Judge Hardy that he was, in fact, tempted by Kay, but that she wisely chose Andy instead. Months later, Judge and Mrs. Hardy are reunited with Andy at Thanksgiving, and are thrilled to discover that he has truly fallen in love with Kay.