Zack gets an anthropology assignment to die for by the new anthropology professor Lasky: conduct a field study of women's wants. Meanwhile, Slater tries to gain weight to be a heavyweight wrestler, while Rogers offers to help Screech as his personal weightlifting trainer.
Zack is invited to a homecoming banquet by the guest of honor, a football hero who could help him score in some business, while Kelly is threatened with come-ons by the alumni. Meanwhile, Slater feels embarrassed being with Alex who shows off her mascot uniform at the banquet and Leslie asks Screech to go with her to shock her visiting snobbish parents.
Zack, Slater, and Screech try to set Rogers up on a date with a lit female professor to get him out of the dorm so they can have a poker game. Meanwhile, Kelly, Leslie, and Alex develop a crush on their new karate instructor, John Hammer, and vie with each other to try to win his affections.
A jealous Zack strikes back at Kelly and Professor Lasky with a vengeance by spreading the word about their May-December affair, which climaxes at a costume ball. Meanwhile, Rogers gives the rest of the gang papers to fill out on psychosis for a project and Screech ends up getting the same mark as Alex.
When Zack finds that Kelly is planning to go on a three-month cruise abroad for the summer, he desperately tries to cling on to her. He tries to make Screech drop out of traveling on the trip so he can take his place. But Zach eventually and makes a big choice to hold onto Kelly: he asks her to marry him. Meanwhile, Slater feels guilty when he sees another girl behind Alex's back since she shares the same interest as he, while Leslie finds out and tries to make him come clean with Alex about his dissatisfaction with their romance.
Lisa Turtle arrives and wants to be the Maid of Honor at Kelly and Zack's coming wedding, as do Leslie and Alex. Meanwhile, Zack and Kelly discus their future as they confront the possibilities of marriage, while Slater is against Zack's marriage plans, while Screech wants to be Zack's best man and throws a bachelor party which goes wrong.