Kishen (Akshay Kumar), a nerd, is unhappily married to Mala (Aishwarya Rai). They have a son, Bunty (Aditya Roy Kapoor), who does not want to get married because his parents are always fighting with each other. His girlfriend, Tanya (Sudeepa Singh), keeps proposing to him but Bunty always refuses.
On their 35th anniversary, Kishen and Mala get in an argument again because of Mala’s friend Kundan (Rannvijay Singh). Kundan hates Kishen because he had been in love with Mala 30 years ago. 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 his laboratory he is working on a time machine. When Anthony Gonsalves asks Bunty why he refuses to get married Bunty tells him that he is scared of marriage because his parents seem so unhappy with theirs. When he returns home Kishen and Mala are fighting but this time it goes too far and they decide to divorce. When Bunty hears this he runs back to Anthony’s lab to use the time machine to go back in time and make his parents fall in love. Without Anthony’s supervision he enters the time Machine and goes back in time.
With the aid of the time machine Bunty goes back to a time before his parents had gotten married. He finds his grandfather’s house where he sees his father, Kishen, and realizes that he is a nerd. Then he sees his mother whom he notices is attractive. Bunty then makes-over Kishen with help from Mala’s friend Mona (Neha Dhupia). While helping Bunty, Mona happens to fall in love with Bunty. Bunty creates a plan to help Mala realise her love for Kishen. Instructing Kishen to ignore Mala while she was becoming friendly and arousing feelings in her, she realises her love for Kishen. But at this point she only tells Mona about her feelings for Kishen However their relatives are against love marriages and therefore do not accept their relationship. So they decide to run away together and get married. Kundan, being in love with Mala, takes Mala’s mother, Bholi Devi (Kirron Kher) and the servant Bhikhu (Rajpal Yadav) with Kishen’s father Rai Bahdur (Om Puri), and chases after Kishen and Mala, who are being helped in their escape by Bunty. After the chase, Kishen slaps Kundan for his bad behavior and everyone is fine with Kishen and Mala’s relationship. Mona then proposes to Bunty but Bunty explains to her that he loves someone else and she is waiting for him.
Bunty then uses the time machine to go back to the future. As expected Kishen and Mala are deeply in love with each other. But unexpectedly Mona is married to Kundan. Bunty then takes Tanya aside and proposes to her and she accepts.