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Gardener Peter Brooks jokes with his fellow worker Joe about the fourth engagement ring his employer's daughter, Joan Madison, has received in two months.  He then writes an insulting diary entry about her in his pocket notebook.  When Peter learns that Joan plans to have his garden work torn up for a lawn party, he complains to her frustrated father that she is a spoiled, pampered, headstrong brat.  After Peter and Joe, working in a tree, spy Joan with a new fawning admirer, Peter drops his notebook, which Joan then reads.  When he states he is keeping records on her for his college thesis on abnormal psychology, she fires him.

The two meet again when Joan visits Bradford College, where Peter is the president of the Spartan Club, a group that seeks through an upcoming vote to change the school back to a male-only college from the coeducational school it has been for five years.  Joan decides to stay to help the girls win the vote, and she encourages them each to snag two or three boys and then withhold kissing and petting privileges until they agree to vote to keep the status quo.

Attempting to make a conquest of Peter, Joan enrolls in his experimental psychology class and arranges for the teacher to select Peter and her as participants in a kissing experiment.  Their kiss, which is preceded by Joan's look of sincere affection, causes both meters attached to them to rise dramatically.  However, Peter then learns that the experiment was rigged and storms out.

That evening, after a meeting during which some Spartans, hungering for female favors, urge Peter to release the proxy votes they earlier granted him, he refuses and leads a parade through the campus.  After he complains in a speech that the admission of girls has ruined campus athletics, Joan challenges the Spartans to a basketball game against the girls.  She encourages her teammates to use sex appeal to distract the boys and the strategy succeeds in the first half, during which Peter does not play because of an injured leg.  With Peter playing, the boys take the lead in the second half before Peter trips over Joan and knocks her out.  Greatly concerned, he carries her off, but when he learns of her stunt, he leaves in a huff.

The night before the vote, Joan and her friends throw a party, during which Joan flirts with a new Spartan.  After stealing his clothes, she sneaks into the Spartan Club and hides in Peter's room.  When he returns, Joan gives him an ultimatum of either giving up his proxy votes or having her be found in his room, which would eliminate his chance to vote.  Peter calls her names, and Joan starts to remove her clothes, which provokes a struggle, just as the dean, tipped off, enters.

The next day, as the balloting is nearly over, the dean informs Peter, who has refused to implicate Joan, that she has confessed the truth about the incident because she loves him.  Although he is now free to vote, Peter dallies until the balloting is over and the girls win.  After Peter and Joan see couples embracing all over the campus, Peter gives her his notebook, which she tosses, and they kiss.