Chicagoan Chester "Chet" Ripley (John Candy), his wife Connie (Stephanie Faracy), and their two sons, Buckley "Buck" and Ben, are vacationing at a lake resort in Pechoggin, Wisconsin, for the summer. All goes as planned until Connie's sister, Kate (Annette Bening), her investment broker husband, Roman Craig (Dan Aykroyd), and their mostly-silent twin daughters, Mara and Cara, unexpectedly arrive uninvited.