Marleau, an out-of-work actor, feels he has found the perfect and easy job: night watchman for a big film studio, Popcorn International. Just as he starts in his new job, a mysterious killer begins doing away with the executives of the company, one by one, signing each of his crime with a lonely trombone tune.What did those poor men do to deserve such a fate? Better for Marleau to find out and quick. Each time, the killer plants a clue to incriminate him!