In The Untouchables, Kevin Costner’s Elliot Ness is stymied by typical Irish police corruption; how, he wonders, can he got after Al Capone if the entire Chicago Police Department is on Capone’s payroll. Then he meets Sean Connery’s Malone:

https://www.youtube.com/watch?v=fs-_7EIxBik

The team Ness eventually puts together comprises: Malone, an almost burnt-out reject cop who is still doing night patrol in his 50s because he won’t join in the corruption; Oscar Wallace, a Treasury Department accountant sent from Washington DC to Chicago to assist Ness; and George Stone/Giuseppe Petri, a police academy recruit who hasn’t even become a full cop yet. 

So when the system is corrupt, it can only be taken down by outsiders of one kind or another. They can be officially part of the system but railroaded and sidelined like Malone; or not from the swamp at all (Ness and Wallace); or still in the young & idealistic phase, not yet tempted by all the enticements of evil power & privilege (Stone).

Here endeth the lesson.