The musical is based around the some of the subcultures of 1950's high school America, including the "tough guy" working-class Greasers from which the musical takes its name.
The movie version sugarcoated the class aspects of the original, and subsequent stage adaptations have played down even further that aspect of the film and emphasised the nostalgia aspects. A sequel, "Grease 2" (1982) was spawned, moving the timeline forward to the early-1960's Kennedy era.
A more dramatic, harder-edged portrayal of some of the same subcultures (though slightly later) can be found in S.E. Hinton's novel The Outsiders[?].
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