The story focuses on Kishen (Akshay Kumar), a well-to-do businessman, who is unhappily married to Mala (Aishwarya Rai). They have a grown-up son, Bunty (Aditya Roy Kapoor), who does not want to get married because of the loveless relationship between his parents. Bunty's girlfriend, Tanya (Sudeepa Singh), keeps proposing to him but he always refuses. On their 35th anniversary, Kishen and Mala get in an argument again because of her friend Kundan (Rannvijay Singh). Kundan had been in love with Mala 30 years ago and still dislikes Kishen. This leads to a fight between Kishen and Kundan. After the fight, Tanya takes Bunty to her grandfather, Anthony Gonsalves (Randhir Kapoor), who happens to be a scientist. When Bunty and Tanya enter Anthony's laboratory, he is working on a time machine.
Anthony asks Bunty why he refuses to get married. Bunty says that he is scared of marriage because his parents seem so unhappy with theirs. When Bunty returns home, Kishen and Mala are fighting again. But this time it goes too far and they decide to get a divorce. When Bunty hears this, he runs back to Anthony's lab to use the time machine and go back in time, so that he can make his parents fall in love. Without Anthony's supervision, Bunty enters the time machine and goes back to a time before his parents got married. He finds his grandfather's house, where he sees Kishen. Bunty realizes that the younger Kishen is a nerd. Then he sees the younger Mala and notices that she is attractive as well as spirited. Mala keeps making fun of Kishen. Bunty makes Kishen over into a 'cool dude' with the help of Mala's friend Mona (Neha Dhupia). While helping them, Mona falls in love with Bunty.
Bunty creates a plan to help Mala realize her love for Kishen. He tells Kishen to ignore Mala, while she is becoming friendlier towards him. The plan succeeds in awakening Mala's feelings and she realizes her love for Kishen. She tells Mona about her feelings. However, Kishen and Mala's parents are against love marriages and, therefore, do not accept their relationship. So the lovers, helped by Bunty, decide to elope. Kundan takes Mala's mother Bholi Devi (Kirron Kher) and servant Bhikhu (Rajpal Yadav), along with Kishen's father Rai Bahdur (Om Puri), to chase the runaway couple. At the end of the chase, Kishen slaps Kundan for his bad behavior. Their parents accept Kishen and Mala's relationship. Mona proposes to Bunty but he tells her that he loves another woman, who is waiting for him. Bunty then uses the time machine to return to the future. As expected, his parents Kishen and Mala are deeply in love with each other. But, unexpectedly, Mona and Kundan are also married to each other. Bunty then takes Tanya aside, proposes to her, and is accepted.