The film narrates the journey of a young man named Raj Sharma (Ranbir Kapoor), who meets three young women at different stages in his life: Mahi (Minissha Lamba), a small-town girl from Punjab; Radhika (Bipasha Basu), an aspiring model in Mumbai; and Gayatri (Deepika Padukone), a non-resident Indian student in Australia. It's the story of a man who goes from one love affair to another, only to run into true love.

In 1996, Raj runs into Mahi on the Eurorail during a Europe trip. Mahi is a dreamy girl, who believes in true love and is sure she will meet her "Raj" (the lead character from the film Dilwale Dulhania Le Jayenge, released in 1995). When Mahi misses her train, Raj, a passenger on the same train, gets off to woo her. He offers to help her reach the airport in time for her return flight. On the way, Mahi falls in love with Raj and they share a kiss after Raj reads her a poem he wrote about his feelings for her. At the airport, Mahi meets her friends and tells them about Raj. When she opens the paper on which Raj had written the poem, she finds it blank. She overhears Raj boasting to his friends about the "stuff" the pair did, and is shocked and heartbroken. Raj realises she overheard and goes away shamefacedly.

In 2002, Raj has moved to Mumbai and found a job with Microsoft as a game designer. He is in a live-in relationship with Radhika, an aspiring model. Raj gets an offer to move to Sydney for a Halo 3 game launch. He expects to be able to leave Radhika and move on, assuming she is a modern girl, who would be able to handle a break-up. However, Radhika unexpectedly announces that she intends to sacrifice her career as a model to marry Raj and join him in Australia. Raj flees by boarding a flight to Sydney on the day he is to marry Radhika. Radhika learns this while waiting for him at the registrar's office, heartbroken.

In 2007, Raj now has a successful career in Sydney. One day, he meets Gayatri, an intelligent, independent girl, who drives a taxi at night and studies at a business school during the day. Raj befriends her and learns that, like him, she does not believe in marriage. However, he realises that he is falling in love with her and proposes. Gayatri turns him down, saying that she is happy with her life as it is. Rejection cuts him deep. As he stands alone watching his true love walk away, he remembers when he broke the hearts of Mahi and Radhika, realising how they must have felt. He decides to seek them out and ask for forgiveness.

On his return to India in 2008, he first seeks out Mahi, who is now married and has two sons. He runs into her husband Joginder (Kunal Kapoor), who expresses his anger at first, as he had heard about Raj. He explains that, while Mahi is the perfect housewife and perfect mother, she no longer believes in love. Raj apologises to Mahi and makes her realise that her husband is the "Raj" she has been waiting for, and Mahi forgives him.

Raj then searches for Radhika. He discovers that she has changed her name to Shreya and has become a highly successful supermodel. After much effort, he manages to meet her and apologises. Shreya refuses to accept his apologies and tells him that if he really wants her forgiveness, he will have to work hard for it. To do this, Raj becomes her personal assistant and she makes him do all sorts of tasks, from cleaning to serving at parties. She tries to humiliate him on every occasion, but Raj remains firm in his resolve and continues to serve without complaint. She finally gives up and tells him how hurt she felt, alone on the day that was supposed to be their wedding. She tells Raj to give up and return home. However, while Raj is waiting for his flight to Sydney, she arrives and tells him how she has realised that the root of her frustration was the bottled-up hate against Raj. She forgives him. Content, Raj returns to Australia, having completed what he had set out to do.

On his return, Raj finds a pile of letters from Gayatri, written over the six months Raj was gone. While he has been away, Gayatri has had second thoughts and regretted turning him down. The lovers reunite and live happily ever after.

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Om Prakash Makhija (Shah Rukh Khan) is a junior artist in the 1970s Hindi film industry, in love with film star Shanti Priya (Deepika Padukone). One evening, Om views the premiere of Shanti's film Dream Girl, posing asManoj Kumar. Later that night, Om becomes drunk and describes his vision of himself as a famous, wealthy film star wining the Filmfare Award for Best Actor to his friend Pappu (Shreyas Talpade) and some local children. At the shooting of a later film, Om rescues Shanti from a fire scene where of the fire has escaped control, and they become friends. One day, after following Shanti, he overhears her and producer Mukesh Mehra's (Arjun Rampal) conversation, that Mukesh and Shanti have married in secret and Shanti is pregnant and expects a traditional wedding. Mukesh pretends to be happy with the news, and invites Shanti at the set of their soon-to-be-made film Om Shanti Om. Mukesh promises to cancel the film, reveal their marriage to the public, and have a grand wedding on the set; but instead traps Shanti on the set and sets it on fire to prevent his financial loss and downfall of his career.

Om, who happens to be at the same location that night, attempts to rescue Shanti, but is attacked by Mukesh's guards, and later thrown from the building by an explosion. Outside, Om is hit by a car owned by Rajesh Kapoor (Javed Sheikh), a well-known actor taking his wife Lovely to the hospital to give birth. At the hospital Om succumbs to his injuries, while Lovely gives birth to a son, also named Om, who in adulthood becomes a popular movie star and a lookalike of Om, nicknamed 'O.K.', and lives the luxurious life imagined by Om Prakash, but experiences pyrophobia and subconsciously inherits Om Prakash's memories. In acceptance of an award, O.K. unexpectedly delivers Om Prakash's drunken speech to the assembly. Pappu hears this on television. At a celebration of his award, O.K. is introduced by his father to Mukesh Mehra, and thereupon completely recovers Om Prakash's memories. He later reunites with Om's mother Bela (Kirron Kher) and Pappu and conspires to avenge Shanti's death by making Mukesh confess his crime. To that end, O.K. convinces Mukesh to restart Om Shanti Om, while scheming to convince Mukesh that Shanti is a ghost haunting himself. To impersonate Shanti, O.K. finds Sandhya, a.k.a. Sandy, a doppelganger of Shanti. Throughout the film's shooting, O.K. and his friends arrange incidents to remind Mukesh of his crime. During the music launch of the film, O.K. taunts Mukesh by revealing, through a song, the extent to which he knows the story of Shanti's death. Seeing Sandy bleed of a scratch, Mukesh confirms that she is not Shanti's ghost. Thereafter O.K. and Mukesh confront each other with their knowledge of each other's actions. During the quarrel, an apparently-genuine ghost of Shanti reveals that Mukesh, after the fire ceased to burn, buried her alive below the set's chandelier. Mukesh attempts to shoot the ghost, shocked by the extent to which she knows about the murder, but Mukesh and O.K. fight, which results in yet another fire; and the chandelier falls on Mukesh, killing him instantly. Pappu and Sandy join O.K. O.K. is shocked that he saw the same person and realizes it is Shanti's ghost. The ghost tearfully bids goodbye to Om. The film ends with Shanti's ghost leaving the scene and disappears in the light.

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