Playing around while aboard a cruise ship, the Chipmunks and Chipettes accidentally go overboard and end up marooned in a tropical paradise. They discover their new turf is not as deserted as it seems.
Kari the babysitter thinks she's in for a night of routine babysitting. She's prepared to provide neurological stimulation with some soothing musical accompaniment for little Jack-Jack, the smallest member of the incredible Parr family. Little does she know that Jack-Jack will teach her a thing or two about babies with "special needs."
Bob Parr has given up his superhero days to log in time as an insurance adjuster and raise his three children with his formerly heroic wife in suburbia. But when he receives a mysterious assignment, it's time to get back into costume.
Through a series of misunderstandings, Alvin, Simon and Theodore come to believe that Dave is going to propose to his new girlfriend in New York City - and dump them. They have three days to get to him and stop the proposal.