Following an accident, Dave is forced to entrust his dear Chipmunks to his cousin Toby, maladjusted and notorious lazy. The latter enrolls them in high school, in order to take advantage of the good time. Alvin and his brothers will therefore have their first comeback: the joys of the canteen, American football, and especially the registration for the interlyée singing competition, which these pop stars intend to win. But that's without counting the arrival of the Chipettes, a deliciously cute girl band who will challenge them on their own ground ...