When a long-lost cousin (David Keith) and his fiancée (Marg Helgenberger) show up at the home of ad executive Richard Sullivan (William Katt) and his wife (Mel Harris), their hosts welcome the pair with open arms. Soon, however, happiness turns to fear as the distant kin begin to take over the Sullivans' lives -- first winning the affection of their son, then going through their effects and eventually assuming their identities.