Harry Brown is not a pleasant film. It is not meant to be. First-time feature director Daniel Barber drops us into a decaying estate where feral youths rule the underpasses with guns, knives and pit-bull bravado, and the police are outgunned, outmanoeuvred and, in one heartbreaking scene, literally too frightened to leave their armoured van. Into this war zone shuffles Harry Brown: pensioner, ex-Royal Marine, widower, and the last man on earth you would expect to matter.

Then they kill his only friend.What follows is less a vigilante movie than a howl of pure working-class rage given human form. Caine, now 76, has never been better. His Harry is quiet, courteous, almost invisible – until the moment the switch flips. There is a scene in a drug den that will make you flinch and cheer in the same breath; Caine delivers it with the flat, exhausted menace of a man who has nothing left to lose and, worse, nothing left to prove.

The film has been accused of reactionary politics – “pensioner Death Wish” sneered some early festival reviews. They miss the point. Harry Brown does not celebrate violence; it stares at it with sickened recognition. When Harry picks up a gun, it is not triumphant – it is tragic.

This is not Charles Bronson grinning under a moustache; this is an old man vomiting after his first kill because he remembers the last time he did this was in Belfast or Aden, and he swore he never would again.Daniel Barber and screenwriter Gary Young refuse to give us the easy catharsis of American revenge fantasies. There are no slow-motion hero shots, no quips. The estate remains broken at the end; the kids are still monsters, the police still powerless, the system still rotten.

The only difference is one more ghost walking the corridors.Emily Mortimer brings fragile decency to a thankless role as the detective who senses what Harry is becoming, and Ben Drew (aka rapper Plan B) is terrifyingly convincing as the gang’s charismatic psychopath.

But this is Caine’s film. He plays Harry with the same south-London accent he was born to, the same exhausted eyes that looked out over Zulu battlefields and Alfie’s bed-sit. When he finally says, almost whispering, “All I know is I don’t know,” you believe every word.

Harry Brown will make you angry, queasy, exhilarated and, if you have any sense, a little bit ashamed. It is not a film that flatters its audience. It is a film that asks what happens to a society that abandons its old, its poor and its decency – and what happens when one of the abandoned finally bites back.Uncomfortable, uncompromising and unmissable.In cinemas Friday. Be warned: you will not feel good when you leave. You will feel awake.