Ivan (voiced by Justin Long), Theodore (voiced by Jesse McCartney) and Simon (Matthew Gray Gubler) are three lovely chipmunks. They have a weird mind, a strong and solid body, and a wisdom that surpasses people. The three of them are very close partners. David (Jason Lee) is the owner of three mice. Recently, he planned to drive to Miami, where his beloved girlfriend lived, and he wanted to propose to her romantically. However, the three chipmunks misunderstood their owner's meaning and thought that Ivan was about to abandon them. Not only that, they also learned that David's girlfriend's son was a cruel "demon"! In order to thwart David's marriage proposal, the three chipmunks came up with a seamless plan.