The short film introduces the audience to the central character, an old man in stereotypical gangster/spiv clothes, who, we are told, is an "amber gambler" - when driving, he always tries to race through the amber light before it changes to red. A voiceover explains how your chances of avoiding an accident statistically decrease each time you do this, and, sure enough, our "amber gambler" is involved in a minor accident. He gets out of the car to confront the other driver, only to find that it is another "amber gambler" who is dressed exactly like him and driving the same kind of car.
This PIF gave rise to the expression "amber gambler" to denote a person who behaves aggressively at traffic lights, although the term is not in common usage today. The PIF was used by Paul Merton to illustrate "pulling out too early" on Room 101, as Michael Winner had nominated the opposite, namely drivers who wait too long at green traffic lights, as one of his most hated things.