Mary Katherine Gallagher (Shannon) is an Irish-American Catholic school girl and social outcast in her high school. She wants to be a superstar so she can get a kiss from Sky Corrigan (Will Ferrell). When Mary is sent to Special Education because of hyperactivity, she makes a new best friend, the tomboyish and equally socially awkward Helen Lewengrub (Emmy Laybourne).
Mary sees her chance to become a superstar when her school promotes a talent show. She wants to try out, but Grandma Gallagher (Glynis Johns) will not let her. Mary tries out anyway; and, when she goes to sign up, a stereotypical cheerleader, Evian Carrie Graham (Elaine Hendrix), gets in a fight with her. The fight causes Evian and Sky to break up, and now Sky is a "single hunk of beefcake on the rebound". Mary is now determined to get in the talent show so Sky will notice her. When Grandma Gallagher finds out that Mary got into the talent show, she finally tells the truth about how her parents died—they were stomped to death while performing in a Riverdance-like competition, rather than being eaten by hammerhead sharks as her grandmother had told her. This is why Grandma Gallagher is against her performing.
However, Grandma Gallagher decides to help Mary do her act in the talent show, as long as she performs for herself. Mary and the other Special Education students spend days practicing. Mary wins the competition as well as Sky's heart. When she kisses Sky though, she discovers he is a horrible kisser and chooses to kiss her friend Eric Slater (Harland Williams) instead.
- Molly Shannon as Mary Katherine Gallagher
- Will Ferrell as Sky Corrigan/Jesus
- Harland Williams as Eric Slater
- Elaine Hendrix as Evian Graham
- Mark McKinney as Father Tylenol Ritley
- Glynis Johns as Grandma Gallagher
- Jason Blicker as Howard
- Gerry Bamman as Father John Insomnic
- Emmy Laybourne as Helen Lewengrub
- Jennifer Irwin as Maria
- Rob Stefaniuk as Thomas Smith
- Natalie Radford as Autumn Winters
- Karyn Dwyer as Summer Falls
- Tom Green as Dylan
- Chuck Campbell as Owen