Tras una serie de malentendidos Alvin, Simon y Theodore creen que Dave va a declararse a su nueva novia en Nueva York, y olvidarse de ellos. Tienen tres días para intentar romper el compromiso y salvarse así de la pérdida de Dave. Through a series of misunderstandings, Alvin, Simon and Theodore come to believe that Dave is going to propose to his new girlfriend in Miami...and dump them. They have three days to get to him and stop the proposal, saving themselves not only from losing Dave but possibly from gaining a terrible stepbrother.