Boy (Chris Klein) meets girl (Heather Graham); Boy falls in love (and has wild, non-stop sex) with girl; Boy loses girl … when, as a result of the ultimate case of mistaken identity, they come to believe they are brother and sister! When the lovesick young man – whom everyone thinks is after some taboo thrills – learns that it really isn’t so, he must race across the country to stop her from marrying another guy.