Welcome to Deeppavali, a crackerjack film with a difference.

Set in North Chennai and mainly in Royapuram, the story revolves around young and dynamic Billu (Ravi), his father Mudaliar (Vijaykumar), his belle Susi (Bhavana) and their friends and lives surrounding them. 

Mudaliar practically owns most of Royapuram - he is sort of a "dada", but he is a good man and takes care of his flock. Billu is his only son, also a good man, full of fire and enthusiasm. He is truly a young hero - unafraid of anything, very respectful and certainly a brave guy without being a goonda. 

Into this neighbourhood, where Mudaliar and Billu reign, comes Susi. Susi is the daughter of Chidambaram (Lall), a biggie in a Bangalore neighbourhood. In Mudaliar's neighbourhood lives Seth (VMC Hanifa) who had worked under Chidambaram a long time ago and Susi is simply visiting. 

Nevertheless, being a small community, Susi makes a lot of friends in the Royapuram area, including Billu and his pals. Susi becomes enamoured of Billu's electric personality and the fire within him and falls in love. Billu too is quite taken by the pretty and sweet Susi and soon, the whole neighbourhood can see their love and accepts them as a couple. 

The story turns in a different direction here. One day, when Billu, Susi and all their friends eat out in a restaurant, in walks Revathy (Minal), spots Susi and happily greets her... only to be told by Susi that she doesn't even know her! Revathy is visibly stumped and upset and walks away! 

Billu and his friends are quite surprised at this conversation and finally, Billu asks Susi seriously what's happening. Susi replies, distressed, that she has a memory loss problem where she is unable to remember people and places from different times since a terrible incident 3 years ago... 

Her father, Chidambaram made a lot of enemies due to his "dada" activities. 3 years ago, someone wanting revenge killed her mother in a hit-and-run incident. Susi's psyche was affected so badly that her memory started giving her problems - she wouldn't be able to remember prior to that horrendous day 3 years ago; if any memories came back, she could even forget what's happening in her present! A diabolical situation indeed... 

In the most poignant manner, she begs Billu to stay by her no matter what tricks her mind plays on her, whether she forgets him one sudden day or not. She asks that he be with her despite her problems. Billu, of course, assures her that he would take care of her for as long as he is alive... 

And as if she had invoked an unpleasant omen, turmoil intrudes into their lives yet again...

Chidambaram gets wind of Susi's and Billu's relationship and is furious. He comes to Chennai, gets his henchmen to beat up Billu and drags Susi away, back to Bangalore. And then, the dreaded happens - Susi, watching the lynching of Billu, faints, only to wake up with her complete old memory from 3 years ago flooding back... and everything about Billu forgotten! ( In computer lingo,she is able to access her archive memory over three years old, but all subsequent memory files and updates are no longer accessible.)

The rest of the story moves in a gripping fashion. Will Billu fight for his love? Does he succeed? Will Susi into accept this man into her heart. a man who is essentially a stranger, thanks to her defective memory? What happens when her father fixes a marriage alliance for her within days of dragging her back to his house - does she walk out with Billu, trusting him?