An honest New York cop named Frank Serpico blows the whistle on rampant corruption in the force only to have his comrades turn against him.
 
This is one of Pacino's best performances and the movie definitely would not be as good if it wasn't for him.  That being said, the film does have some dragging parts and Al's wardrobe (especially his hats) can be distracting from time to time, but his acting always pulls you back into what's going on in the film.
 
FUN FACT:  The film was shot in reverse order. Al Pacino began with long hair and a beard, then for each scene, his hair and beard were trimmed bit by bit until he became clean-cut.