When an uninvited and unwelcome family suddenly arrives to join their relatives for a summer of relaxation, the results are anything but restful. Chet Ripley's (John Candy) plans for a quiet vacation at a lakeside resort quickly take a turn when his obnoxious brother-in-law, Roman Craig (Dan Aykroyd), and his snooty family appear unannounced and ready to join for a season of "fun" in the sun. Unfortunately, the only thing these two in-laws have in common is their intense dislike for each other. Soon, it's brother-in-law against brother-in-law in a fight to the finish!