Yedhunandhan(Vignesh), a pious, devout Brahmin who is orphaned at an early age, comes to the city to look for a job. Mistakenly arrested, he ends up working at the police station itself and becomes a good friend of the inspector. It is here that he meets Mangai(Divya), who lives with her grandmother and wants to become a policewoman. Owning up to a crime he didn't commit to help out the inspector, Yedhunandhan goes to prison and after being released, hooks up with Mayakka(Vadivukkarasi). Mayakka is one of the people working for Thevar(Nasser) and Yedhunandhan becomes a member of her group. But discontent among Thevar's employees leads to trouble.

For a while, Aachaarya treads a different path. In gangster movies, we are used to seeing the protagonist being gradually changed by his surroundings. But here's a protagonist who manages to change those around him. He may have come into contact with them under tough circumstances and he may be aligned with someone for whom violence is a way of life. But he remains the same person he always was and lives his life the way he has always done. This rubs off on the people he works with and they treat him with the respect he deserves. So he always has a positive effect on the places he steps into.

But once Vignesh takes up violence(the reason behind this is very predictable too) the movie enters familiar territory and begins to resemble recent films about rowdies who are at the lower levels of the gang hierarchy. We see Vignesh being dealt a tough hand as he realizes that he(and others like him) are expendable and we see him go on the run with no one to trust or turn to. This realistic look at a rowdy's dangerous life may have been fresh a year ago but after a line of movies that have tackled the same topic, Aachaarya just looks dated.

But the movie does differentiate itself from other movies of the same ilk by its tone. It maintains a sober, serious tone throughout and rarely gives in to commercial temptations. Comedy, barring some humorous comments by Ganja Karuppu, is largely absent and songs, all of which are slow numbers, are heard only in the background. Even the romance is handled differently. The relationship between Vignesh and Divya is handled rather awkwardly initially and seems like an unnecessary add-on but it justifies its existence later.

The realistic tone continues right upto the climax, which is unexpected and sudden. But that also proves to be its undoing. Things happen so suddenly that the film is over before we realize what has happened and so the events, though surprising, lack drama. The end credits start rolling before the full impact of what has happened sinks in and so the ending feels rushed and not fleshed out enough.

Vignesh is one of those actors who's been around forever hoping for the right break. He does the job here and is convincing as both the good-natured young man and the rowdy. Divya looks rather ordinary and doesn't make much of an impression. Vadivukkarasi is impressive as Mayakka. She is introduced as a loud-mouthed woman who controls her market with an iron hand but is soon shown to have a good heart. The actress brings out this contrast convincingly. Nasser, with a moustache reminding us of his turn in Thevar Magan, is solid as usual.