Rock band No Vacancy performs at a nightclub three weeks before auditioning for the Battle of the Bands. Guitarist Dewey Finn creates on-stage antics, including a stage dive that abruptly ends the performance. The next morning, Dewey wakes in the apartment he lives in with his best friend and former rocker Ned Schneebly. Ned and his domineering girlfriend, Patty inform Dewey he must make up for his share of the rent, which is four months overdue. When Dewey meets No Vacancy at a rehearsal session, he discovers he has been replaced by another guitarist named Spider. Later, while attempting to sell some of his equipment for rent money, Dewey answers a phone call from Rosalie Mullins, the principal of the Horace Green prep school, inquiring for Ned about a short-term position as a substitute teacher. Desperate for money, Dewey impersonates Ned and is hired. On his first day at the school, Dewey adopts the name "Mr. S" (after not actually knowing how to spell Schneebly) and spends his first day behaving erratically, confusing the class.