Venkatramakrishnan (Srikanth), Sevalkodi Senthil (Jeeva) and Panchavan Parivendan (Vijay) are three engineering students who share a room in the residence of Ideal Engineering College (IEC), Chennai. Venkat is studying engineering to pursue his father's wishes over his own wish — to become a wildlife photographer. Senthil is studying to raise his family's fortunes and get them out of poverty. On the other hand, Pari studies for his simple passion in machines and devices. Pari believes that success in studying is not achieved by solely memorizing definitions given by textbooks or professors, but by understanding and applying the concepts. He believes that one should follow excellence, not success, as success will come itself if excellence is followed.
However, this different approach is sneered upon by the faculties, including the dean of the college, Professor Virumandi Santhanam "Virus" (Sathyaraj). In contrast, Virus' favorite student, Srivatsan "Silencer", (Sathyan) believes in mindless memorizing over understanding, in order to reach his goals of corporate and social status. Meanwhile, Pari falls in love with Virus' daughter who was a medical student Ria (Ileana D'Cruz) when he, Senthil and Venkat accidentally crash her sister's wedding banquet to obtain a free meal, which infuriates Virus. Virus labels Pari and his friends as "idiots" and continually attempts to break Pari's friendship with Senthil and Venkat.
Things further escalate when the three friends drunkenly break into Virus's house one night to allow Pari to confess his love to Ria, and urinate on the front door before fleeing. The next day, Virus threatens to rusticate Senthil unless he squeals on Pari. Not wanting to betray his friend or let down his family, Senthil attempts suicide and ends up paralyzed. Following his recovery, Senthil discards his fear of the future. Encouraged by Pari, both Senthil and Venkat gain courage to do what they were afraid to do before. Senthil takes a frank approach in an interview for a corporate job, while Venkat convinces his parents to allow him to pursue his love of wildlife photography, both successful in their endeavors.
When Virus discovers that Senthil successfully obtained a job offer, he conspires to prevent him from getting the job by creating an exam paper so tough that will deliberately fail Senthil. When Ria learns of his plan, she helps Pari and Venkat by providing them with the keys to her father's office. However, Virus catches them and expels them on the spot. Ria angrily confronts him, revealing that his son, Ria's brother, committed suicide when he could not get into IEC, like Virus demanded, even though he had no interest in engineering and instead wished to be a writer. At the same time, Ria's pregnant older sister Swetha (Anuya Bhagvath) goes into labor. A heavy rainstorm cuts all power and floods the streets, making it impossible for the ambulance to reach Swetha, and Ria is stuck at her hospital, where she had fled. Pari, Senthil and Venkat come to the rescue, along with other students from IEC. Over phone and webcam, Ria instructs Pari to deliver the baby via VOIP. After the baby is apparently stillborn, Pari resuscitates the baby by leading the students in saying "All is well". Virus reconciles with Pari and his friends, and allows them to stay for their final exams.
Their story is framed as intermittent flashbacks from the present day, ten years after Srivatsan bet that he'd become more successful than Pari. Having lost contact with Pari, who disappeared during the graduation party and went into seclusion, after five years, Senthil and Venkat begin a journey to find him. They are joined by Srivatsan, now a wealthy and successful professional in the USA, who is looking to seal a deal with a famous scientist and prospective business associate named Kosakhi Pasapugazh. When they find Pari's house in Ooty, they find a completely different man: the real Pari (S. J. Surya).
From him, they come to know that their friend was a destitute servant boy "Pappu" who loved learning, while he, the real Pari, disliked study. After seeing the boy's intelligence, the family agreed to let the servant boy study in Pari's place instead of laboring. In return, the real Pari would pocket his qualifications and after graduating, the servant boy will cease all contact. The real Pari reveals that Pappu is now a schoolteacher in Dhanushkodi. At the same time, they find out that, without Pari, Ria decided to marry the same price-obsessed banker that Pari initially talked her into dumping, and they go to rescue her from the wedding in Coimbatore.
Senthil and Venkat find Ria and arrive at Pari’s school. Ria and the fake Pari rekindle their love, while Srivatsan mocks Pari for becoming a lowly school teacher. He asks Pari to sign a "Declaration of Defeat" document. When Pari's friends ask what his real name is, he reveals that he is actually Kosakhi Pasapugazh himself. Srivatsan finds out about this and is horrified; he accepts his defeat and pleads his case with Kosakhi Pasapugazh to establish the business relationship he was after.
· Vijay as Kosakhi Pasapugazh (Panchavan Parivendan)
· Jeeva as Sevalkodi Senthil
· Srikanth as Venkatramakrishnan
· Ileana D'Cruz as Ria Santhanam
· Sathyaraj as Virumandi Santhanam (Virus)
· Sathyan as Srivatsan (Silencer)
· S. J. Surya as Panchavan Parivendan (Guest appearance)
· Anuya Bhagvath as Swetha Santhanam
· Vijay Vasanth as Paneerselvam
· Indrans as Virus's assistant
· Manobala as 'Librarian' Bose
· Uma Padmanabhan as Venkat's mother
· S. Shankar in a special appearance
· Ajay Rathnam as Panchavan Parivendan's father