Gwen Cummings (SANDRA BULLOCK) and her boyfriend Jasper (DOMINIC WEST) are the hard partying types who obviously have addiction problems. Showing up drunk to be a bridesmaid at her sister Lily's (ELIZABETH PERKINS) wedding, Gwen caps off her embarrassing appearance by destroying the wedding cake and then driving a limo into someone's porch.
Rather than be sent to prison, Gwen is enrolled at Serenity Glen, a rehab center where the patients must go through a twenty-eight day program of accepting that they have a problem, participating in group meetings to discuss those problems and living substance free for nearly a month. Of course, Gwen doesn't think she belongs there or that she has a problem, and doesn't like the way that head nurse Betty (MARGO MARTINDALE) or her counselor and former addict himself, Cornell (STEVE BUSCEMI), treat her.
Nor is she particularly thrilled with her new set of "friends" that includes her roommate, Andrea (AZURA SKYE), a teenage junkie who's been through rehab before, as well as Bobbie Jean (DIANE LADD), a Southern housewife; Roshanda (MARIANNE JEAN-BAPTISTE) whose addiction has troubled her kids; Daniel (RENI SANTONI), a former doctor whose addiction cost him his practice; Oliver (MICHAEL O'MALLEY), who's always hitting on Gwen; and Gerhardt (ALAN TUDYK), an insecure and gay German stripper.
When Jasper shows up for a little R&R from rehab, Gwen returns to the center drunk and/or high, thus breaking their rules. As a result, Cornell orders that she be removed from rehab and sent to prison to serve out her time. Facing that consequence, Gwen realizes she has to clean up her act and decides to participate in the center's activities.
As she tries to do so, she must deal with her cravings, Jasper's repeated attempts to relive their former lives, her relationship with her sister, and her partial attraction to fellow rehabber Eddie Boone (VIGGO MORTENSEN), a professional baseball pitcher who arrives at the center not only with his own problems, but words of wisdom for her as well.