Maariyappan aka Maari is a fun-loving gangster whose sidekicks are Sanikkizhamai and Robert aka Adithangi (punching bag). Maari is chased around by Araathu Aanandi, an auto rickshaw driver who is head over heels in love with him. However, try as she might, Maari does not give her a second glance and constantly shows his annoyance to her.

Meanwhile, Beeja aka Thanathos has escaped from prison, after killing two officers, with the cell walls scrawled with the words Kill Maari. He is out for vengeance as his brother, who was a defence lawyer, promised to get Beeja released from jail. Unfortunately, his brother is killed at the hands of Maari as the former tried to molest Aanandi's elder sister. This is also the reason why Aanandi loves Maari. While coming back after a party, Maari gets attacked, Aanandi takes him to the hospital and takes care of him. Kalai's brother Vallavan is shown to be hand-in-glove with Singayya, who has formed an alliance with Beeja to overthrow Maari. As per Beeja's plan, Vallavan asks Sani to send Aanandi for a job, which turns out to be drug smuggling. Aanandi unknowingly goes to do the job and gets caught by Kalai. On confronting Aanandi, she tells him that Maari asked her to do it. This sparks a doubt for Kalai. Vallavan is caught by Maari carrying drugs, and when Maari confronts Kalai, a fight erupts. Maari and his sidekicks set fire to all the drugs, and Beeja kills Vallavan to make it look as Maari killed him. Kalai gets enraged and orders his men to kill Maari. Maari is injured in the battle, and Beeja reveals himself as the perpetrator and shoots Maari, but Maari is saved by Aanandi, who took the bullet for him, which ended up in her spinal cord, paralysing her for life.

The new District collector, Vijaya Chamundeshwari, comes out to restore order and stop all gang violence. Maari goes into hiding with Aanandi, whose legs are paralysed. Eight years pass, Beeja and Kalai have grown to become rich dons, but when Kalai has a clear chance to become the MLA candidate of the ruling party, Beeja usurps that chance by threatening Kalai and his family. Vijaya, now the Deputy-Home secretary, in an effort to find Maari, posts his picture along with Aanandi's on newspapers. Maari meets Vijaya, requesting her to remove the photos. Maari then reveals that he was living in Tenkasi with an elderly couple. Aanandi got pregnant, and a healthy baby boy was born. However, due to complications by the paralysis, Aanandi dies minutes after childbirth. Maari then raises the boy, called Kaali with help from the old couple, Sani and Robert. Kaali is shown to have inherited Maari's behaviour and keeps picking fights in school. Maari then requests Vijaya to stop coming after him as he changed his ways. When Kaali sees some students teasing a teacher, he complains about them to the principal. In retribution, the boys chase and attack him, only to get thrashed by Kaali. The council's area member, who is also father of one of the boys, calls Kaali to hit him, but Maari goes and begs for forgiveness.

Meanwhile, the city police commissioner mentions to the media that there is an informer within Beeja's gang. Using his moles in the police department, Beeja tries to figure out who the mole is but fails. It is then revealed that there was not any mole and that the whole story was fabricated to identify the corrupt policemen in the department, including the commissioner. Also, Beeja has been in ties with the commissioner and was the mole himself. He had been leaking info about his men so that he may come out clear to become an MLA candidate. It is also revealed that despite begging for forgiveness, the counsellor insisted on his son beating up Kaali, and so, Maari beats up the counsellor and his henchmen. In a final encounter, the police turns against Beeja. Maari fights Beeja and paralyses him for revenge for Aanandi and reveals to him that Maari killed his brother because Beeja could be killed by his own brother for wealth. Soon Beeja gets sentenced to life term prison. Maari is then shown with his sidekicks continuing his old life and also reunites with Kalai.