An amoral and unprincipled - but charismatic and good-looking - young man calling himself Jack turns up to a Halloween party dressed as the Devil. The lady of the house is Kim Buckley, a beautiful but shy young woman who is vulnerable to the flattery of others, despite being married to a wealthy business consultant. Jack calmly announces he's the illegitimate and unacknowledged son of Kim's recently deceased father. She had no idea he existed.
Feigning a brotherly interested in Kim, he is actually out to seek revenge on her for being the pampered only daughter of their rakish father. Things are complicated by the fact they have a mutual powerful sexual interest in each other and, although Kim is filled with shame and guilt at this unnatural obsession, she finds herself being drawn into his gameplan for success.