The story focuses on Kishen (Akshay Kumar), a well-to-do businessman, who is unhappily married to Mala (Aishwarya Rai). They have a young 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 rejects. She takes Bunty to her grandfather, Anthony Gonsalves (Randhir Kapoor), who happens to be a scientist and 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.
On their 35th anniversary, Kishen and Mala get in an argument because of her friend Kundan (Rannvijay Singh). Kundan used to tease Kishen and tries to repeat it after all these days. Mala doesn’t stop him, moreover laughs over Kishen along with her friends. This leads to a fight between Kishen and Mala and it goes too far that they decide to get a divorce. When Bunty hears this, he runs 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 realises that the younger Kishen was an unattractive, weak and socially awkward youth where as younger Mala was attractive as well as spirited. Mala keeps making fun of Kishen. Bunty makes Kishen over into a ‘cool dude’ with the help of money he borrowed from younger Anthony. Mala’s friend Mona (Neha Dhupia) falls in love with Bunty.
Bunty creates a plan to help Mala realise her love for Kishen. Meanwhile, Kundan also falls for Mala, making things more complicated. Bunty tells Kishen to ignore Mala, while she is becoming friendlier towards him. The plan succeeds in awakening Mala’s feelings and she confesses 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 Bahadur (Om Puri), to chase the love birds.
At the end of the chase, Kishen slaps Kundan for his bad behaviour. 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 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 she accepts happily.