Sly plays a smalltown cop in a corrupt county (probably not unlike the one many of us live in). He's willing to look past some things, but eventually he gets fed-up and fights the system which includes several dirty cops.
This isn't the great Rocky who can whip his weight in lions and tigers. He's just a regular middle-aged guy with a beer belly, a gun, and a night stick doing the best he can. This should have been the start of a whole new movie career for Stallone--a chance to really make quality movies aimed at adults. Unfortunately, he soon fell back into the run-o-the-mill Rocky/Rambo type of action movies that do well at the box-office, but have little to offer by way of heart and insight into the workings of the world. I, for one, hope he makes more films like Cop-Land in the future.