Wayne (Mike Myers) and Cassandra (Tia Carrere) break up after she views his pics he took while spying on her during her lunch date with her manager Bobby (Christopher Walken), and soon Cassandra begins to date Bobby and eventually becomes engaged to marry him and poised to leave Chicago for Los Angeles. Garth (Dana Carvey) breaks up with Honey (Kim Basinger) after learning she was not completely divorced from her husband, and foregoes her request to kill him.
Wayne tracks down Cassandra and prevents her from going through with the wedding, evading Bobby and her father while they high tail it to Waynestock, Wayne's concert. When they get there, they learn from Garth that none of the bands have arrived. They are then transported to the desert and meet with Jim Morrison. Garth tells him the bands haven't shown up, and he says they won't. When Wayne asks why he put on the concert in the first place, Jim says he had to do it to try and do something with his and Garth's lives, and although it was a failure, they had to be glad that they tried anyway and that they gave it their best shot. He apologizes, and as he walks away with the Weird Naked Indian (Larry Sellers) who says that they'll think of something because it was their movie. Wayne & Garth sarcastically thank him and proceed to go through the desert alone.
As they go across the desert, nearing death, they decide to refuse to end the film that way, and to go with the Thelma & Louise ending. When that ending doesn't work, they decide upon the happy ending, taking them back to the Waynestock concert at the moment Garth tells Wayne none of the bands have arrived. Aerosmith then shows up in a limo and proceed to go on stage and sing "Shut Up And Dance." The other acts soon follow.
Garth hooks up with Betty Jo (Olivia d'Abo), the girl who bears an uncanny resemblance to him, and Wayne learns from Jim away from the concert that the reason behind making Waynestock happen was that it didn't matter what he did because Cassandra loved him for who he was, and that being an adult means facing responsibility but yet still taking the time to have fun.
Thanks Tornado Dragon!