The film starts with the police arresting Aadhikesavan's (Suresh Gopi) men suspecting them for burning an MLA's wine shop. The MLA sends a false evidence to the court against Aadhikesavan. But Aadhi sends Dheenadhayalan(Ajith Kumar) who is actually his adopted brother to attack the false evidence. Later some people complain to Aadhi that the MLA had removed the breakwire from a schoolvan due to rivalry between his school and their school leading to the death of many children. Aadhi asks Dheena and his henchmen to cut the hands of the MLA and his men especially the right hand and Dheena completes his task.
Meanwhile, Dheena falls in love with a girl named Chitra (Laila) who initially mistakes him to be a bus conductor and later a vegetable seller. One day one of Dheena's henchmen ('Mahanathi' Shankar) spots Chitra and her cousin in the plaza and reports to Dheena. Dheena pretends to meet her by chance and pays their bill using the plaza manager's money. One day Chitra calls Dheena's home landline number posing as 'Pepsi' Uma and comes to know about Dheena's tricks. Meanwhile, Dheena comes to know that someone (Shyam Ganesh) has teased his sister Shanthi and thrashes him. One day when Dheena meets Chitra they are ridiculed by a gang of rowdies but Dheena stays calm as he thinks that girls hate people who are violent. The next day he meets Chitra at the plaza and tells him he should have beaten the rowdies as she likes violence. As a coincidence the rowdies come and Dheena thrashes them and informs Chitra that he is a local goon. Later she comes to know that Dheena has love for her as she did. Meanwhile, the same person who teased Shanthi gave a love letter to her and Dheena beats him up and comes to know that the persons sister was Chitra. Chitra tells him that they both love each other and Dheena agrees to help them. Shanthi accepts this to Dheena but afraid to tell to Aadhi. Finding the difficulty of the situation,Shanthi elopes with her lover and Aadhi's men chase them in which they get into an accident. In the hospital Shanthi tells Dheena to protect her lover's family from Aadhi. Dheena promises and she dies.
Aadhi,enraged by his sister's death asks Dheena to kill the family of the boy responsible for Shanthi's death. But Dheena refuses and tells that Shanthi was in love with that boy. But Aadhi could not believe this and misunderstands Dheena was refusing since he loved the boy's sister Chitra. Aadhi swears that he will kill Dheena also and declares he is not his brother anymore.
Dheena moves to Chitra's flat to protect her family but her brother does not believe him. Aadhi kills one of Chitra's brother's friend and lands in jail. Dheena stays in the room of two auto drivers (Maadhu Balaji and Sriman) who believe in him. One time Aadhi sends his men to kill Chitra's family but Dheena stops them and in the process he gets badly wounded. Chitra's family starts believing Dheena and their love develops.
Later Aadhi gets released and orders his men to kill Dheena and Chitra's family. The MLA Malarvannan (Bala Singh), who was his enemy, wants to kill Dheena, but Aadhi asks him to stay away due to ego. Malarvannan creates confusion by sending half of his men in the outfit of auto drivers and remaining in outfit of Aadhi's men. Dheena finds this and goes to warn Aadhi but is unaware that one of the Malarvannan's men had stabbed Chitra. Meanwhile, Aadhi comes to know about Shanthi's love from her college friends. Aadhi and Dheena join forces and stop the confusion. Knowing that Malarvannan was responsible for this, Aadhi goes to kill him, but Dheena stops him that if he kills him, another person would avenge and this chain could continue and asks him to leave violence. The end is shown in the hospital where Chitra wakes to see Dheena and Aadhi. Aadhi finally declares Dheena to be his brother hereafter.Ajith Kumar as Dheenadhayalan (Dheena)
Suresh Gopi as Adhikesavan
Laila Mehdin as Chitra
Vijayalakshmi as Chitra's Mother
Bala Singh as MLA Malarvannan
Vaishnavi as Chitra's friend
Dhadi Balaji as auto driver
Sriman as auto driver
'Mahanathi' Shankar as Narayanan
Nagma in an item number
The film's director A. R. Murugadoss had previously worked as an assistant to S. J. Surya and signed up Ajith Kumar and Suresh Gopi to play the lead roles with Laila in the female lead. Nagma was selected to appear in an item number.
The film received a positive review from The Hindu, with the critic stating that the "acts of the hero does appeal or impress the audience because his bloody adventures in the film are justifiable" labelling the film as a mass entertainer. Rediff.com suggested that "overall, the film is briskly paced, and holds your interest throughout. What prompts the 'Can do better' report are the little, but eminently avoidable, niggles." but praised the performances of Ajith Kumar and Suresh Gopi. Another critic said, "Dheena, while not offering anything new, plays it safe by offering something for everybody. A story that involves a nice conflict, a cute romance, well-choreographed dance sequences, nicely-staged fight sequences and humorous dialogs(targeted mostly at the college guys) are the recipe for this entertaining mixture".
The film took a big opening at the box office and went on to become a commercially successful venture.
The soundtrack was composed by Yuvan Shankar Raja and released on 23 December 2000. The songs proved to be very successful upon release and established him slowly in the Tamil film industry. Lyrics were written by Vaali.
The film's success indirectly marked the beginning of a new image of Ajith, that of an action hero that would appeal to the masses. Furthermore, Ajith's nickname in the film, Thala (Leader) became a new identity for the actor amongst his fans. The film also established Murugadoss who went on to become one of the successful directors whereas Yuvan Shankar's songs were equally successful which are considered to have played a major role for the film's great success.