Jumping forward a few years Serpico has turned plain-clothes, mainly because he's keen and he wants to be a detective (to get the gold shield). It turns out that he's a natural for this line of work - he looks scruffy, grows a moustache and acts nothing like a policeman. This approach gets results but alienates him from his colleagues, since they can't cope with anyone just slightly different. Serpico is generally happy though, with lots of friends (outside the force), a nice flat and lots of girlfriends. The only irritation is the police corruption - it's still there and now it's even bigger. It seem that all of the plain-clothes division are on the take, extorting money from petty crooks. The pressure on Serpico to take his cut is intense but he's both honest and stubborn - he just won't give in.
Unfortunately, with this atmosphere something has to give eventually; Serpico knows that his non-conformity is preventing his promotion and affecting police work generally. With an ally, from his early days as a police officer, Serpico tries to raise the problem of corruption with those in authority, with predictable consequences. Since the people in power have risen through the same system they're not interested in causing trouble and damaging the reputation of the force. It seems that, despite the overpowering stench of corruption, they're happy as long as Serpico keeps his complaints internal. Serpico is very much alone in this situation - if he'd just take the money everybody would be happy - and all he can do is keep applying for transfers. With this slow build-up of frustration and anger it's obvious why Serpico is, at this moment, lying in a hospital bed. What's less clear is how did he get there?
This story relies totally on the characters who populate the crime-filled streets and Pacino, as the central figure, doesn't let us down. He is convincing as the honest, decent, individual and unyielding pawn stuck in the middle of an explosive situation. That Serpico in no way wants an investigation and yet feels compelled to expose the widespread bribery is shown with wonderful subtlety. Together with solid supporting performances, the result is an exciting and believable portrayal of life on the front-line. A further bonus is that all of the filming took place on location in NYC, with the grime and sleaze of the big city as a backdrop to the story. (A further nice touch is that Serpico buys a sheepdog puppy at the start of the film, which then grows older and bigger with him and provides an element of continuity).