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.

A co-production between Canada and the United States, the film was primarily shot in Montreal, with some additional shooting taking place in New York City.

[4] 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.

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.

He spends his lunch break at the library researching rat behavior, and although he is horrified by the revelations, he shares them at the dinner party that evening, ruining the appetites of the other guests.

Unshaven and disheveled, Bart approaches Eliot in the lobby of the office building, and declares that his first priority is to address his troubles at home.

He dreams of a happy reunion with his family, interrupted by the rat attacking Meg, and Peter accidentally ingesting poison.

As Bart regains consciousness, the creature descends from the ceiling, forcing him to take refuge on a hammock suspended above the bedroom floor.

Donning leg and arm pads, and reinforcing his baseball bat with nails and the jaws of broken rattraps, Bart enters the basement to face his nemesis.

[3] The majority of the film was shot on a 7,000 square foot set, recreating the interior of a Manhattan brownstone, created by production designer Anne Pritchard.

"[10] Kevin Thomas of the Los Angeles Times praised the film, describing it as "a visual tour de force... Of Unknown Origin is just fast, taut and darkly comic enough not to seem preposterous.

[12] Peter Weller won the Best Actor Award at the 1983 Paris International Festival of Fantastic and Science-Fiction Film, for his performance, with director George P. Cosmatos winning the Grand Prix prize for Best Feature.