Music manager Dave (Jason Lee) is not only enjoying a burgeoning career, but a blossoming romance with Samantha (Kimberly Williams-Paisley), a beautiful doctor who works in the emergency room. Through a series of misunderstandings, the chipmunks start to believe that a marriage proposal is on the horizon. Dreading the thought of an antagonistic stepbrother (Josh Green), Alvin (Justin Long), Theodore and Simon embark on a cross-country odyssey to prevent their best pal from ruining their lives.