Of Unknown Origin is a 1983 psychological horror film directed by George P. Cosmatos, and starring Peter Weller, Jennifer Dale, Lawrence Dane, Maury Chaykin, and Shannon Tweed. Based on the 1979 novel The Visitor by Chauncey G. Parker III, it focuses on a mild-mannered Manhattan banker who becomes increasingly obsessive and destructive in his attempts to kill a rat loose in his renovated brownstone. The film's title refers to the misconception (repeated in the film) that rats have no known origin.

Plot

Bart Hughes, a mild-mannered investment banker in New York City, moves with his wife Meg and their son Peter into a brownstone he helped to renovate. Meg's wealthy father invites the family to a vacation in Vermont, but Bart declines, preferring to work on a project that should get him a promotion. Shortly after his wife and child leave, Bart learns that the project has been assigned to another employee, James Hall. Bart is outraged, until his boss, Eliot Riverton, assigns him the important task of writing a reorganization plan for the company’s branch offices, due in two weeks. Eliot also invites Bart to join him at a dinner party for the bank's Los Angeles branch manager the following Thursday.