Arunachalam is a story about a village hero (Rajnikanth) who has 2 brothers and a sister and a happy family. Ravichandran and K.R.Vatsala are his parents. Vedavalli (Soundharya) is the daughter of Jaisankar and Venniraadai Nirmala. Nirmala is Ravichandran's sister and they all come to Ravichandran's home for Rajni's sister's wedding. That gives the opportunity for Soundharya and Rajnikanth to fight first, then to kiss and fall in love with each other.
Rajnikanth finds himself to be an orphan by interval. He goes out of the village. He goes to Madras where he meets Janakaraj, who gives him a job. By some magical situation Rajni discovers himself to be a son of a (dead) millionaire - through Visu. Visu is the guardian of Rajni's father's trust. If Rajni wants his inheritance, he has to follow certain rules. He can get an inheritance of 3000 Crore Rupees if he can spend 30 crores in 30 days. The conditions are: no contributions to charities, not to own any assets at the end of 30 days and above all, no one else should know about this! Receipts should be provide for all the expenses. If Rajni fails to spend the money according to the rules, all the money will go to his father's trust which is managed by V.K.Ramasamy,

Nizalgal Ravi, Kitty and Raguvaran. These four form a team of Villains;. They want to spoil Rajni's efforts and get all the money for the trust and kill Visu. Ramba is Visu's daughter and se is appointed as the accountant for Rajni for 30 days.

What happens next is the main part of the story. Rajni tries hard to spend his money and many times, the money comes back! He spends on Horse races, lottery tickets, makes a movie with Senthil as the hero and he even starts his own political party with Janakaraj being the candidate. All these events are good opportunities for situation comedies.
Does Rajni win this competition? Does he get all the 3000 Crore rupees for himself and live happily ever after? (see it in silver screen!) is the rest of the story.