I'm a big sucker for vigilante movies, but too many of them end up with the vigilante either getting killed or regretting his actions, which is pure cuckery in my book. Not this movie though. The basic premise is that a friend of an old army veteran is killed by a gang of criminals, and since the bong police is filled with cowardly, politically correct faggots (which the movie both acknowledges and mocks) the protagonist decides to take matters into his own hands.
There's no intricate plot or surprising twists, it's a by-the-book vigilante/revenge story, the twist here is that the crusader for justice is an old man, and not the kind Hollywood likes to present. He's old in the real sense of the world, he's not going to fight some young thug barehanded, he's not going to perform amazing parkour stunts, all he's got going for him are his wits and his military experience, and he uses them to full effect.
Another trope this movie has, and that I always delight in seeing, is the stronk womyn diversity hire getting put in her place, and boy does this movie deliver in that regard.
So, to summarize, it's about an old guy that unrepentantly goes after criminals and punishes them for what they did to his friend, it mocks British police for the cucks they are and it does so doubly for the female police officer, showing her as both incompetent and hopelessly naive.
If you have even a slight interest in vigilante movies do yourselves a favor and watch Harry Brown.