During the first day back of the new school year at J.F.K. Junior High, preppy and pathological liar Zack Morris runs into a 9th grade student, named Karen, to whom he had lied about his age earlier that summer in order to get to know her better. He persuades classmates Mikey, Screech, Tina, and Nikki to help with his charade. Meanwhile, Miss Bliss tries to squeeze a dinner date into her new schedule with her new boyfriend Brian, the doctor.
Screech asks Zack to write a love note for him to give to Lisa in exchange for writing his term paper. Miss Bliss inadvertently discovers the note and general chaos ensues when she thinks Zach wrote it for her. Then, Mr. Belding finds it and thinks Miss Bliss wrote it for him, then Tina finds it, and so on leading to general chaos.
After Zack breaks his Dad's $300 video camera, he asks Mikey to help him scam enough money to buy a new one before his father finds out. When Miss Bliss current history topic is the stock market, Zack sees a chance to making it big and deceives Miss Bliss into investing a high-risk stock involving potatoes and when the stock drops, it costs Miss Bliss a lot of pocket money.
Nikki refuses to do Mr. Morton's science class which includes dissecting frogs. So she, aided by Zach, Lisa, Mickey and Screech releases them, which makes things worse instead of better. Meanwhile, Mr. Belding tries to be friendly to Miss Bliss after mistakenly thinking that she has received an offer for a teaching position at another school.