Three chipmunks, Alvin, Simon and Theodore, watch as their home is chopped down by a Christmas tree company. With the boys still inside, the tree is shipped off to the lobby of the prestigious Jett Records, helmed by a slick and conniving record executive named Ian Hawk (David Cross), in downtown Los Angeles. Ian is a former college roommate of Dave Seville (Jason Lee), a down-on-his-luck musician and songwriter who has never given up hope for making a successful career out of his quirky musical sensibilities.
When Dave goes to Jett Records to pitch Ian a new song, he is rudely booted out of the offices. Taking a basket of muffins as consolation, he wanders past the Christmas tree being installed in the lobby. The Chipmunks, smelling the tasty muffins, jump in the basket unbeknownst to Dave, who unwittingly carts them back to his apartment where the Chipmunks secretly install themselves quite comfortably.
Before you can say, "Alvinnnnn!!!" the 'Munks run amuck, turning their new digs into a disaster area. The boys' home-wrecking antics include storing toaster waffles under the carpet (put aside for the winter, the 'Munks explain), and leaving some mysterious dark, round objects strewn about (Simon insists they're...raisins). Shocked at what the boys have done to his apartment, Dave is even more astounded when he discovers that not only can these rodents talk, they can sing.
Despite his misgivings about his new tenants, Dave seizes the opportunity to put together his songwriting skills with the 'Munks unique talents.