The story is about a pensioner vigilante. This may explain why half the audience at the theatre were senior citizens!
Michael Caine plays Harry Brown, a former (highly decorated) British Marine -- as Harry says, that was a lifetime ago. In his retirement, Harry holes up in his apartment in a thug-infested part of London, careful to avoid any confrontation with the roving youth gangs. His courage is lacking.
When he visits his wife (who's in the final stages of Alzheimer's) in the hospital, he's careful to avoid a subway tunnel where the gangs often hang out. His only recreation is to go to the local pub to play chess with his one and only friend, Leonard. Leonard is fed up with these street gangs. They're always hassling him.
When Leonard decides to carry an old army bayonet with him for self-protection, Harry admonishes him. Harry fears that Lenny will get himself killed.
Alas, his fear comes true. When Lenny is killed, Harry is left all alone in the world (his wife died a week earlier). Harry reaches his breaking point and decides to wreak justice...
(A side note: at Lenny's funeral, there was only the priest and Harry. It was a very sad image. For some reason that I cannot quite fathom, I worry about no one showing up at *my* funeral. This is totally illogical -- when I'm dead, why the frak should I care who shows up???)
Harry makes for a sad vigilante. He has emphysema, so it's sadly comical that he wheezes and stumbles while pursuing his prey.
Emily Mortimer plays Detective Inspector Alice Frampton who's on the trail of the vigilante. She was the one who informed Harry of Lenny's death, and eventually she connects Harry with the string of killings in the neighbourhood. The police brass doesn't believe her -- they find the notion of a pensioner vigilante absurd.
Michael Caine gives one of his very finest performances. I highly recommend this movie.