Nachiappan (Nasser) owns an Indian restaurant in Los Angeles. His twin sons Vishwanathan or Visu and Ramamoorthy or Ramu (Prashanth in dual roles) are medical students who also work at the restaurant, along with the chief cook, Juno (Senthil).
While at the airport on some work, Visu notices some fellow Indians, Madhumitha or Madhu (Aishwarya Rai), her brother Madhesh (Raju Sundaram) and their grandmother Krishnaveni (Lakshmi) in trouble. He helps them and comes to know that Krishnaveni has to undergo a critical brain surgery. Krishnaveni's family gets confused with Ramu and Visu appearing at different times and finally gets surprised to know that they are identical twins. Krishnaveni's cousin Sivasu, who agreed to play host doesn't help and Krishnaveni is annoyed.
Krishnaveni is keen to visit the city and enjoy it as she is not confident about the success of the surgery and Ramu takes her on a city tour. She goes for the operation but does not recover. Visu, an intern in the same hospital visits her and notices that she has been operated on the wrong side of her brain. Visu appeals aggressively to the doctors and gets another surgery done and also gets a compensation of $2 million for the family. Krishnaveni recovers quickly.
While departing, at the airport, Krishnaveni realizes that Visu and Madhu are in love. She cancels the trip pretending to be ill and extends their stay there. Nachiappan objects to Krishnaveni's marriage proposal of Madhu with Visu as he wants his sons to marry identical twin girls because he himself has an identical twin brother, Pechiappan (Nasser). His flashback is revealed...
They both had married two women who were not sisters. Pechiappan's wife Sundaramba (Radhika) showed such tyrannical behaviour towards Nachiappan's wife (Geetha), that the latter died delivering the twin boys when Nachiappan was away. Nachiappan is disheartened and decides to leave with his newborn babies to USA. Nachiappan's mother-in-law says that he should have listened to her advice of twin brothers should marry only twin sisters so that they will support each other.
Krishnaveni solves the problem by lying to Nachiappan that Madhu has an identical twin, Vaishnavi. Ramu immediately starts liking the imaginary Vaishnavi. The entire family comes to Chennai to meet the family: father Vellaiappan (S.Ve. Shekar) and mother Durgadevi (Janaki Sabesh). Velliappan is shocked to know that his mother has lied and Krishnaveni tells them that she will manage. She convinces Madhu to act like Vaishnavi for a couple of days.
Krishnaveni manages to project Madhu as Vaishnavi, a very demure, traditional girl and Ramu falls for her act. Juno is the only one who doubts this. Confusions galore as both Visu and Ramu start showing their affection to Madhu. When Nachiappan desires to see them both, Madhesh uses digital technology and manages to show them both. On the day of engagement, Nachiappan puts forth a demand that both the marriages should take place together.
That same day, Pechiappan arrives to meet his brother Nachiappan and he is warmly welcomed. Pechiappan attempts suicide and the Police rescue him. He shares his sad story with his brother and Nachiappan comes up with a plan. They switch places and Nachiappan goes as Pechiappan to Karaikkudi to reform his brother's wife. He renovates the hotel business, which attracts many customers. The villagers advise Nachiappan to remarry as Sundaramba cannot have a child and he refuses stating that he will live only with her in this lifetime. Hearing this she reforms.
Visu visits Madhesh's technical room and finds out the truth. Visu shouts at Madhu for cheating him and Ramu. Madhu tries to apologize and Krishnaveni tries to take the blame but he does not agree. She tries talking to Pechiappan, who is pretending there as Nachiappan, who does not understand the happenings.
Overnight Ramu reconciles and convinces Visu to accept Madhu. Ramu also persuades Pechiappan, impersonating his father, to get Madhu and Visu married. Ramu states that Pechiappan and his wife are the reason for all these problems. Pechiappan feels guilty and agrees and advances the wedding date out of fear that his brother will stop the marriage.
At Karaikkudi Sundaramba is reformed. Nachiappan plans to take her to the marriage. By then, Madhesh, who left the city when the problems began at home, meets Nachiappan and thinking he is Pechiappan seeks his help by telling the truth. Nachiappan is shocked and rushes with Madhesh and Sundaramba to the marriage hall. He stops the marriage and Ramu is confused and demands to know how he can go back on his word after agreeing for the marriage. Nachiappan now understands that it was his brother Pechiappan who had given the consent. Both the sons start accusing him as a cheat. Pechiappan now intervenes and tells the truth about them switching places. Sundaramba is shocked to know the truth and now, accuses Nachiappan of cheating on her and expresses her desire to remove the 'thaali' (sacred thread). Nachiappan justifies that impersonating is not wrong when done with good intentions. Sundaramba now states that, if that is the case, what Madhu and her family have done is also not wrong as it was done to unite the lovers. Nachiappan realizes the situation and Vellaiappan convinces him to think about his children's happiness and Nachiappan agrees and Visu marries Madhu.
[from the book Pride of Tamil Cinema: 1931 to 2013, by G Dhananjayan, Blue Ocean Publishers, 2014]