along with 'the french connection' this is one of the classic copthrillers.
steve mcqueen plays a luitenant who's assignment collapses with the murder of a key witness and who fights the extorsion put on him by a senator. he makes his own way as a cop and resolves a scam-operation. the streets of san fransisco are the playground for a hectic carchase, mcqueen keeps/plays his cool, and together with a good story you get a highly entertaining movie.