The story revolves around two best friends, Meera and Nithya with contrasting personalities. Meera is an extrovert while Nithya is an introvert. They have been friends since childhood and work in the IT industry in Chennai. They live as roommates and have a group of friends in their apartment. A handsome young man Venkatraman, moves into a unit in their apartment and all the young women in the neighborhood are smitten by him, but Meera and Nithya choose to ignore him initially. As time goes on, Meera is attracted to Venkat when he handles some issues in their apartment with his wisdom and good nature. She makes a phone call to appreciate him and eventually, through repetitive phone calls and long telephonic conversations, they become friends and their friendship blossoms into love without meeting in real life. Nithya understands this development with Meera's relationship status as well. Unable to deal with Meera's elusiveness any longer, Venkat plans to meet her face to facefor the first time and decides to surprise her by walking to her apartment while she is speaking on the phone with him. But in the last minute, Meera gets distracted by another call on her mobile and thrusts the landline phone into Nithya's hands and walks away to attend it. Venkat walks into the apartment and mistakes Nithya for Meera and forcefully kisses her to express his love and leaves the place. Nithya is shell-shocked and is unable to speak anything for a while. Meera is oblivious to all this because the other call was a family emergency and she leaves soon after, failing to notice what happened to Nithya. Over the next few weeks, Nithya suffers alone at home after falling sick with fever due to shock and refuses to attend further calls from Venkat. He decides to visit her, still under the impression that she was the one speaking to him all this while. When he finds out about the fever, he apologizes for his behavior but doesn't listen to her when she tries to tell him about the mistaken identity. He takes good care of her as she recovers, but Nithya is stricken with guilt at her inability to make him understand the truth. Soon, he shocks her once again by introducing her to his family, friends and colleagues who tell her about his good character and that she is a lucky person to have found Venkat. The same night Venkat proposes to marry her. Nithya is overwhelmed by all the emotions and happiness from the well wishers and is unable to refuse his proposal or reveal the truth. So she decides to play along and accept this new turn of events in her life. When Meera returns from her hometown, Nithya is unable to face her. She asks Meera to forget Venkat saying that he was not serious about her and is just playing around. Meera refuses to believe Nithya's accusations and challenges that Venkat himself will meet her very soon and she will wait for him to do so. Meanwhile, Nithya plants a false impression about Meera to Venkat that she is a possessive friend who is trying to spoil her relationship with him, so he must not believe anything she tells hereafter. Soon, Venkat gets a chance to go abroad for his job and manages to secure a job for Nithya as well. While trying to convey this information and about their upcoming wedding to Nithya, he accidentally meets Meera who thinks that Venkat is back for her and is on cloud nine assuming that he really loves her. To add fuel to the fire, a harmless conversation from Venkat has Meera assuming that the wedding plans are for herself. Nithya is unable to bear the guilt of betraying both her friend and her fiancé and decides to tell the truth before they marry and leave for the States, but Meera meets with an accident and is critical at the hospital. Nithya confesses everything to an unconscious Meera and swears that she does not want to cheat her and marry Venkat. The day of the wedding arrives and Meera gains consciousness and goes to meet Nithya and Venkat. Nithya is ready to give up Venkat for Meera, but she refuses and forgives Nithya and willingly sacrifices her love for her friend. She says that they deserve each other and asks for Nithya's promise to keep this secret between themselves. Nithya and Venkat get married and a few years later we find that Meera has died as an unmarried woman, still holding the memories of her love for Venkat.
The film score and soundtrack were composed by Yuvan Shankar Raja, who teamed up with director Sabapathy Dekshinamurthy for the first time. The soundtrack released on 3 November 2002 and features 8 tracks with lyrics penned by Arivumathi, Pa. Vijay, Pazhani Bharathi and M. Rathnakumar. For the first time, composer Yuvan Shankar Raja made an appearance in a film, by featuring in and performing one of the songs, "En Kadhal", which was sung by himself as well. One additional song, "Idhu Innisaiya", sung by Bollywood singer Hema Sardesai and Yuvan Shankar Raja, featuring Bollywood actress Sarika on screen, was not included in the soundtrack. ;Not in the soundtrack Other music featured in the film include:
"Idhu Innaisaiya" - sung by Hema Sardesai & Yuvan Shankar Raja, composed by Yuvan Shankar Raja
Malathi Rangarajan of The Hindu in her review stated "The story offers nothing new. Tales ofunrequited love and mistaken identity have been seen on screen for years now. It is the treatment that makes the difference in "Punnagai Poovae"."
