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Harry is a widower living in a house near a rather rough apartment complex in London. He sees the thuggish kids who hang out at the nearby underpass, and he avoids them by taking the long route. People in the neighbourhood are harassed, fed up and on edge. Harry can feel the bad vibes building up around him but it's not until an old friend of his tries to defend himself and ends up dead that he realises that he can't just sit by and watch. Coincidentally, Harry happened to have been in the Marines as a young man. Those skills might come in handy.
Soon, Harry is on a mission to clean up the 'hood, while a well meaning but ineffectual cop (Emily Mortimer) who started out investigating his friend's death starts to suspect him of the recent spate of gang-member deaths.
There's not much to this film other than Michael Caine's amazing acting chops and some clever action sequences, but that's totally good enough to make it 97 minutes of awesome. Caine is a much more three dimensional and frail vigilante than Eastwood's Kowalski. There's depth to his old-guy-badass, and it makes for some great action. Of particular note is an extended gun purchasing scene in which the tension is so drawn out that when it finally breaks, it's hard not to let out a cheer.