Aadhavan (Suriya) is a professional assassin working with his father Ibrahim Rowther (Sayaji Shinde) and brother Tharani (Anand Babu). Doctor Abdul Kulkarni (Rahul Dev) hires Aadhavan to kill prominent Judge Subramaniam (Bharath Murali), since he is handling the inquiry and verdict on Abdul’s child kidnapping, rape, murder, and organ trafficking cases in Kolkata. Aadhavan attempts to murder Subramaniam but fails at the first attempt. After Abdul insults him for his inability, Aadhavan is angry with himself and promises to kill Subramaniam within ten days. He threatens the Judge’s comical servant Bannerjee (Vadivelu) to help him get into the house as Murugan, Bannerjee’s brother in law, while he locks up the actual Murugan (Sathyan) in his ship.
Slowly Aadhavan begins to win over the members of Subramaniam’s household – his mother (B. Saroja Devi), her beautiful granddaughter Thara (Nayantara) and his other relatives. Thara is attracted to Murugan’s (Aadhavan) intelligence and simple-innocent nature. Aadhavan tries to assassinate the judge at home in the middle of the night and also plants a bomb in Subramaniam’s van, but both attempts fail. Meanwhile, Subramaniam’s entire family is on a trip to Darjeeling, and Thara asks Aadhavan to hide a guitar belonging to Ilaiyaman (Ramesh Khanna), her madcap musician cousin. Aadhavan uses this opportunity and plants a mobile bomb in the guitar and places it in one of the cars which explodes at the wrong time and he fails to kill Subramaniam yet again. Meanwhile, a security officer at Subramaniam’s house keeps informing Abdul Kulkarni of Aadhavan’s entry into the house and all his progresses and failed attempts.
Thara is now suspicious about Aadhavan but when she questions him in secret, he informs her that he is really Subramaniam’s long lost runaway son Madhavan. Thara soon informs all the members of the household about Madhavan and they are overjoyed and Thara and Aadhavan fall in love. However, they hide it from Subramaniam and Ilaiyaman, because of his hatred towards Madhavan. Aadhavan, however tells Bannerjee that he lied to Thara about Madhavan to distract her suspicion, and that he really is not that runaway son at all. However, on Thara’s birthday, a flashback reveals that Aadhavan is indeed the runaway Madhavan.
Years ago, a ten-year-old Madhavan unknowingly carries a gift with a hidden bomb for his cousin Thara’s birthday. His friend’s mother plants the bomb in the toy to avenge Subramaniam for her husband’s jail sentence. Though Thara survives, her mother Anu (Anu Hassan), opens the box and is killed along with her husband in the explosion. Subramaniam blames Madhavan for his sister Anu’s death and for his friendship with a gangster’s son and thrashes him. In anger and fear of going to a juvenile prison, Madhavan steals a gun from the cops and accidentally shoots his father and runs away from home. He kills his friend’s mother as she is the responsible for his aunt’s death. He saves a local goon, Ibrahim Rowther from death, and is in turn adopted as his son and rechristened as Aadhavan.
Coming back to the present, Tharani tried to kill Subramanium, he is stoped by Aadhavan and Aadhavan reveals his truue identity that He is the son of Subramanium and Ibrahim Rowther challenges within two days I will kill you and the judge. . Abdul Kulkarni is fed up with the repeated failed attempts of Aadhavan. So he makes another plan and threatens Ibrahim and his gang to surrender to the police and make a confession about Aadhavan and his plans. When the Assistant Commissioner of police (Riyaz Khan), confronts Aadhavan and is about to have him hauled off, Aadhavan gets Subramaniam’s laptop containing the verdict and inquiry reports, takes him and Thara as hostages and drives away. In the meanwhile, the police commissioner (FEFSI Vijayan) arrives and makes a confession that the police actually had caught him after his very first attempt to kill the Judge had failed. When questioned by the commissioner, Aadhavan said that he backed off and misfired on purpose at the last moment, after he found that he was going to kill his own father. The Police then ask him to go back to the house to protect his father over a twenty-four-hour period everyday. This information is relayed to the Judge who is tearfully grateful to his son. Meanwhile, Abdul Kulkarni attacks the vehicle in which the trio are travelling and a fight ensues where Aadhavan saves his father and Thara, and Kulkarny is killed.
As the credits roll, we see that Madhavan and Thara are now married and live in the household with the huge family, and yet again Bannerjee brings two assistants (K. S. Ravikumar and Udhayanidhi Stalin), to help him and asks approval from Madhavan and he gladly approves.