Alvin, Simon, and Theodore are living contentedly with their caregiver, Dave, who has been dating Samantha for some time. The trio is less than thrilled about Dave's relationship, especially due to Samantha's troublemaking teenage son, Miles. When they discover an engagement ring in Dave’s bag, the Chipmunks panic at the thought of losing Dave and having Miles as a stepbrother. They hatch a plan to disrupt the proposal while on a trip to Miami, believing it will shift Dave's attention back to them.

The Chipmunks and Miles, through a series of misunderstandings and adventures, find themselves inadvertently bonding on their way to Miami. Meanwhile, Dave remains unaware of the chaos that ensued from his well-meaning boys. As the four travel, they face numerous humorous obstacles and mishaps but learn valuable lessons about family and friendship. As they near their destination, the journey becomes one of self-discovery and acceptance, culminating in a pivotal moment that redefines their relationships and understanding of what truly matters in their lives.