Venkat Ramakrishnan (Srikanth), Sevarkodi Senthil (Jiiva) and Panchavan Parivendan (Vijay) are three engineering students who share a room in the hostel of Ideal Engineering College (IEC), Chennai. Venkat studies engineering to pursue his father's wishes over his own wish — to become a wildlife photographer. Senthil studies to get his family out of poverty, while Pari studies engineering for his simple passion in machines and devices. Pari believes that one should follow excellence, not success, as success will take care of itself if excellence is followed. However, this different approach is sneered upon by the the dean of the college, Professor Virumandi "Virus" Santhanam (Sathyaraj). Virus and the faculty favour Pari's classmate Srivatsan (Sathyan), also known as "Silencer", believes in mindless memorizing over understanding in order to reach his goals of corporate status. Pari constantly spars with Virus and Srivatsan due to their differences in their educational approaches.

Meanwhile, Pari falls in love with Virus' daughter Ria (Ileana D'Cruz), a medical student, when he, Venkat and Senthil meet her when they accidentally crash her elder sister Swetha's (Anuya Bhagvath) wedding reception to obtain a free meal. Ria is not impressed with Pari's antics and complains to her father about them. Virus becomes furious with Pari and his friends, labels them as "idiots" and tries to break their friendship, by telling Senthil and Venkat that Pari is a bad influence on them. While Venkat refuses to break his friendship with Pari, Senthil believes Virus and parts with Pari. However, when Pari saves Senthil's father's life, who had suffered a heart attack, by taking him immediately to the hospital where Ria works on the latter's scooter when the ambulance had not come, Senthil becomes grateful to Pari and reconciles with him. Ria too appreciates Pari's practicality and concern for others and falls in love with him. Later, in the semester examination, Pari tops his batch, proving that the his approach to studies does succeed.
3 years later, Pari, Venkat and Senthil break into Virus's house in a drunken state one night to allow Pari to propose to Ria. Venkat and Senthil cause a ruckus and urinate on the front door before fleeing. Virus notices Senthil and the next day, he threatens to rusticates him unless he snitches on Pari. Not wanting to betray his friend or let down his family, Senthil attempts suicide by jumping from Virus' office window and ends up paralyzed. Following his recovery, Senthil discards his fear of the future. His frankness impresses the corporate agents during a campus interview and they hire him. Venkat, goaded by his friends, convinces his parents to allow him follow his wildlife photographer dream.

Virus later schemes to set an impossibly tough final exam to fail Senthil, as the job offer is contingent on graduation. When Ria learns of her father's plan, she helps Pari and Venkat to leak the question paper by providing them with the keys to her father's office. However, Virus catches them and expels them on the spot. Ria angrily confronts her father, revealing to him that his son (Ria's brother) wanted to be a writer, but was forced to apply for engineering in IEC because of him. As he could not get selected there, he committed suicide. At the same time, Swetha, who is pregnant, goes into labour.

A heavy thunderstorm cuts all power and floods the streets, making it impossible for the ambulance to reach Swetha, and Ria is stuck at the hospital, where she had gone for work. Pari, Venkat and Senthil, along with other students from IEC, deliver the baby using the available equipment and the inverter developed by Pari, with Ria instructing him via VoIP. After the newborn baby is apparently stillborn, Pari manages to resuscitate the baby. Virus forgives Pari and his friends, and allows them to write their final exams. Pari eventually graduates as the "Most Outstanding Student" in his batch. After the convocation however, Pari disappears and goes into seclusion.

7 years later, Venkat and Senthil set out on a journey to Ooty to find Pari. They are joined by Srivatsan, who is now a wealthy and successful professional in the USA and is looking to seal a deal with a famous scientist and prospective business associate named Kosaksi Pasapugazh. Srivatsan's main intention in finding Pari is to see whether Pari is more successful than him; they had made a bet 10 years before on who would be more successful with their study approaches. However when they reach Pari's house in Ooty, they find a completely different man: the real Panchavan Parivendan (S. J. Surya).

Confronting the real Pari, they find out from him that their friend was actually a destitute servant boy named Pappu. Pappu, who worked in Pari's household, loved learning, while Pari disliked studies. After seeing the boy's intelligence, Pari's father (Ajay Rathnam) agreed to allow Pappu study in Pari's place instead of working as a servant. In return, Pari would pocket Pappu's qualifications and after graduating, Pappu will cease all contact with Pari and his family. Pari reveals that Pappu is now a schoolteacher in Dhanushkodi. Later, Venkat and Senthil find out that as "Pari" had disappeared, Ria had decided to marry her former boyfriend- a price obsessed banker whom she had dumped when "Pari" made her realize that he loves his expensive items more than her. Venkat and Senthil rescue Ria from her wedding in Coimbatore and they along with Srivatsan drive to Dhanushkodi.

At "Pari"'s school, Venkat and Senthil reunite with their long-lost friend, Ria and "Pari" rekindle their romance, and Srivatsan mocks at "Pari" for becoming a lowly schoolteacher. He asks him to sign a "Declaration of Defeat" document for losing the bet. "Pari" signs the document and, but when Srivatsan realizes that the fake Pari's real name is actually Kosaksi Pasupugazh. Srivatsan accepts defeat and begs Kosaksi to sign the business deal, which the latter apparently refuses.

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