Alvin has recently been having nightmares of meeting the Wolfman. Simon and Dave conclude that Alvin's been watching too many horror films at night. Alvin says that it is because their new neighbor, Larry Talbot, creeps him out and speculates that he is hiding something. When yet another accident caused by Alvin when mixing unknown chemicals results in a huge explosion and mess of the school auditorium, along with Dave getting a call from Miss Miller about the Chipettes being scared after being spooked by something while walking home with the Chipmunks one night, Principal Milliken and Dave decide that Alvin be pulled out of his role as Mr. Hyde in their school play for Dr. Jekyll and Mr Hyde, and is given the role of the butler instead with Dave confiscating all of Alvin's monster paraphernalia. To boost his self-esteem, they decide to have Theodore replace the role. Millikin expects Theodore to start standing up for himself when Dave sees Nathan, a bully, causing him trouble.Alvin and Simon begin searching for proof that Mr. Talbot is a werewolf. Theodore gets bit by a large dog (though it was actually a werewolf) on his way home after giving a necklace to Eleanor, who he has a crush on. The next day at the rehearsal, Theodore does an impression of Mr. Hyde. Despite this triumph, Theodore transforms into a puppy-like werewolf by night, and his personality continues to drastically change by the day. With their brother cursed with Lycanthropy, Alvin and Simon search for a way to help Theodore and save the play without Dave finding out. Despite their best efforts, they find no solution. They eventually decide to take up advice from psychic Madame Raya. She says that Theodore is close to the animal state and will turn into a werewolf soon. Simon and Alvin ask her if there is any way to cure him. She suggests knocking him out with a silver cane, before the next full moon when the transformation will be complete. Alvin later breaks into Mr. Talbot's home and steals his silver cane, which is then broken in two by Theodore after Alvin knocks him with it, but Theodore isn't cured. However, as he runs away with it, he knocks into Dave. That night, Dave goes to see Mr. Talbot to apologize and explain everything to him. During the conversation, in which Talbot mentions an ancestor killed by a silver bullet, the full moon rises and he transforms. Terrified, Dave runs to the school to warn the boys. However, he's knocked unconscious by a pole. Having followed Dave, Mr. Talbot makes his way inside and chases after Alvin (who realizes he was right to suspect Mr. Talbot).During the play, Theodore turns into a werewolf while and attacks Eleanor. However, after cornering her, the necklace Theodore gave her earlier shines by the moon, causing Theodore to remember his feelings towards her and to flee. Eleanor follows him, determined to help him, only to almost be attacked by Mr. Talbot, who was the werewolf that bit Theodore. Theodore quickly defends her and attacks Mr. Talbot and the two battle. As a result of the bite, Theodore turns back into a chipmunk and Talbot returns to normal. Talbot then runs off the stage. Confused by what happened, Simon explains to everyone how the bite cured them by causing the effect to reverse on them both. Alvin quickly runs up to the stage to join in the applause by the crowd, who believes the entire incident was just an act. At the wrap party, they find out Mr. Talbot is going to be their new principal as Millikin is taking a more challenging job driving a truck filled with nitroglycerin. Having woke up, Dave shows up as he gives his speech about his new position and nearly attacks him as he starts to claim that Talbot is a werewolf. The boys quickly explain that Theodore took care of everything, leaving Dave impressed and proud of him. As Dave hugs Theodore, Mr. Talbot whispers, "Thank you." The Chipmunks and Chipettes end the wrap party by doing their famous performances, and soon everyone else follow the rhythm as the movie ends.