Bayside's 40th birthday celebration is in full swing. Bobby is running for homecoming ""king"" against Rachel, who's running for queen. A millionaire alumnus, J. Walter McMillan, comes to tour the school. He plans to buy Bayside and tear it down for condo construction. The gang are all reassigned to different schools. Screech's work program will come to an end and Mr. Belding will be looking for a job. Screech and the kids resolve to stop McMillan. Screech disguises himself as attorneys for both sides and the gang chain themselves to lockers, but nothing works. The school is slated to be destroyed after the prom.

The students are depressed about Bayside's closing and can't get very excited about the prom. Mr. McMillan shows up and says he now plans to tear down the school even before the prom is held. Desperate to stop him, Rachel is disguised as ""Nurse McGillicuddy"" and the kids fake an outbreak of ""Swiss measles,"" for which a phony quarantine is set up. The prom is back on. The gang finds out that Mr. McMillan was a lonely guy at Bayside, with only one friend, a girl named Sandy Bennett. They locate Ms. Bennett but she can't come to Bayside's 40th prom because of a previous engagement with her nephew. The prom has a 1959 theme. Former Bayside High students Lisa Turtle and A.C. Slater show up, then Zack Morris arrives with Ms. Bennett, who it turns out is his aunt. McMillan is reunited with his old friend and she convinces him to abandon his plans to tear down Bayside.