The story revolves around three roommates, journalist Tashi (Imran Khan), photographer Nitin Berry (Kunaal Roy Kapur) and cartoonist Arup (Vir Das), leading an unkempt and debt-ridden life in a shoddy apartment. Tashi's ditzy fiancée, Sonia (Shenaz Treasurywala), is an air hostess who agrees to deliver a package for Vladimir Dragunsky (Kim Bodnia) to Somayajulu (Vijay Raaz), without realizing its contents or that Somayajulu is a gangster. Sonia asks Tashi to deliver the package. Tashi, in turn, asks Nitin to do so. But Nitin is unable to do so as he is suffering 'Delhi Belly'. Nitin hands Sonia's package to Arup for delivery to Somayajulu, along with a package containing his stool sample for delivery to Nitin's doctor. Arup mixes up the two bags. Somayajulu, furious, tortures Vladimir to find his package.
The film begins in a bowling alley, where a group of friends are awaiting the arrival of Salim. Meanwhile, Deepak arrives with his fiancée Sheetal. When a conversation about love arises, Deepak and the rest find it necessary to make Sheetal aware that true love does exist. They tell her the story of Raj (Shah Rukh Khan) and Priya (Rani Mukerji), two of their closest friends. Raj Mathur is the owner of Raj Transport, a small trucking company he founded himself. Raj is a carefree man who is unorganised, messy, lazy and late all the time. Although he isn't the richest, he's always happy.
L'histoire vraie de l'hôtesse de l'air de 22 ans, Neerja Bhanot (Sonam Kapoor), tuée à Karachi en 1986, lors du détournement du vol 73 Pan Am, dans lequel elle était chef de cabine, alors qu'elle sauvait la vie de 360 passagers.
Le film est inspiré de la vie d'Arunachalam Muruganantham, un entrepreneur indien du Tamil Nadu, qui a inventé une machine permettant de fabriquer des serviettes hygiéniques à bas coûts après avoir constaté que les femmes de son village utilisaient des chiffons usagés en guise de protection hygiénique.
Quatre amies originaires de Delhi, Kalindi (Kareena Kapoor) qui est sur le point d'épouser son petit-copain, Rishab (Sumeet Vyas) de longue date malgré sa phobie de l'engagement, la très accomplie, Avni (Sonam Kapoor), qui souhaite désespérément se marier et enchaîne les rendez-vous amoureux et autres rencontres arrangées, Sakshi (Swara Bhaskar) dont le mariage rencontre des difficultés et Meera (Shikha Talsania) jeune maman qui s'est mariée sans le consentement de sa famille, se réunissent à l'occasion du mariage de Kalindi .
Sidhu (Akshay Kumar) is a lowly vegetable cutter at a roadside food stall in the Chandni Chowk section of Delhi, who consults astrologers, tarot card readers, and fake fakirs despite his foster father Dada's (Mithun Chakraborty) exhortations. When two strangers from China claim him as a reincarnation of war hero 'Liu Shen' and take him to China, Sidhu, encouraged by trickster Chopstick (Ranvir Shorey), believes he will be feted as a hero, unaware of his own recruitment to assassinate the smuggler Hojo (Gordon Liu).
Maya (Rani Mukerji) is an orphan who is to marry her closest childhood friend Rishi Talwar (Abhishek Bachchan). The two have been raised by Rishi's fun-loving father Samarjeet a.k.a. Sam (Amitabh Bachchan). Dev Saran (Shah Rukh Khan) is a successful soccer player in the United States. He lives in New York with his wife Rhea (Preity Zinta), a fashion magazine editor, his young son Arjun (Ahsaas Channa) and his mother Kamaljeet a.k.a. Kamal (Kirron Kher). Dev encounters Maya moments before she is to marry Rishi, and the two have a deep conversation about love, life and happiness. Although Dev and Maya are strangers, they connect instantly. When Dev bids Maya goodbye, she tells him they should never say goodbye because it kills the possibility of meeting again. Moments after they part, Dev is hit by a car and permanently injures his leg; as a result, he is no longer able to play soccer.
Reshma (Vidya Balan) is running away from her house to Chennai, a day before her marriage. She is insulted by the casting director for being unattractive and useless after trying to get a role in a film. Determined to convince him she spontaneously grabs the role of a side dancer. However, she dances with whips and using erotic movements which annoys the film's director, Abraham (Emraan Hashmi). He edits out Reshma's entire dance sequence from the film. The film fails at the box office much to the dismay of the producer Selva Ganesh (Rajesh Sharma), who later offers Reshma a song in his upcoming film, and suggests that she now be referred to as "Silk".
Durga Singh (Raakhee) is a poor woman raising her two beloved sons Karan (Salman Khan) and Arjun (Shahrukh Khan). Her sons give her bangles, which she says she shall give to the women they marry. Later, when the Munim of an aging Thakur comes to talk to Durga, Karan and Arjun learn that their father is dead. Durga reveals that their father was the Thakur's son, who had married her against his father's wishes. Durjan Singh(Amrish Puri), a Relative of Thakur, killed Durga's husband to prevent him, Durga, or their sons from inheriting the Thakur's estate. Karan and Arjun learn from her that the dying Thakur wants to give his estate to his grandchildren before dying as it is their birthright.
Five-year-old Krishna Mehra undergoes an intelligence quotient test by a professor, who suspects that he has superpowers. His grandmother Sonia (Rekha) takes the young Krishna to a remote mountain village to conceal his unique abilities. Years later, Krishna (Hrithik Roshan) meets Priya (Priyanka Chopra) and her friend Honey (Manini Mishra) when Krishna's friend Bahadur (Hemant Pandey) takes the girls' camping crew near his home to camp. Krishna saves Priya from a hang glider accident and falls in love with her when spending time with her.
The story follows Aarush, (Akshay Kumar), a 'Panauti' who works in bar and gets money as people lose games of poker. After being rejected from getting married by a girl he goes to London to stay for some time with his best friend, Bob (Ritesh Deshmukh) and his wife, Hetal (Lara Dutta), both working at a casino owned by Mr.Kishore Samtani (Randhir Kapoor). Although at first Hetal is displeased with Aarush's presence, he proves to be a kind-hearted man looking for family. Bob and Hetal decide to get him married to their boss' daughter, Devika Samtani (Jiah Khan). After the marriage, on their honeymoon in Italy, Devika leaves him for her long-time American boyfriend Benny. Aarush decides to commit suicide by drowning in the sea but is saved by a Telugu girl Sandy (Deepika Padukone), who he kissed when she was trying to make him breathe.
A poison-gas attack on a Kolkata Metro Rail compartment kills the passengers on board. Two years later Vidya Bagchi (Vidya Balan), a pregnant software engineer, arrives in Kolkata from London during the Durga Puja festivities in search of her missing husband, Arnab Bagchi. A police officer, Satyoki "Rana" Sinha (Parambrata Chatterjee), offers to help. Although Vidya claims that Arnab went to Kolkata on an assignment for the National Data Center (NDC), initial investigations suggest that no such person was employed by the NDC.
The film opens by showing a large crowd waiting for Rahul Jaykar (Aditya Roy Kapoor) – a successful musician whose career is waning because of his alcohol addiction – to perform at a stage show in Goa. After nearly completing a song, he is unexpectedly interrupted by Aryan (Salil Acharya), who was losing his career due to Rahul's, during his performance. Rahul fights him, stops his performance, and drives to a local bar. He meets Aarohi (Shraddha Kapoor), a bar singer who idolizes Rahul. After noticing Aarohi looking at a photograph of Lata Mangeshkar in the bar, he assumes that she wants to become a singer. Impressed by her simplicity and voice, Rahul promises to transform her into a singing sensation, and asks her to never perform again in bars. Aarohi leaves her job and returns to Mumbai with Rahul, who convinces record producer Saigal (Mahesh Thakur) to meet her. When Aarohi calls Rahul, he is attacked and injured by some thugs, and is unable to receive her call. His friend and manager Vivek (Shaad Randhawa)says that news of Rahul's accident should not be leaked to the media, and instead publicises a false story that Rahul has left the country to participate in stage shows. When Aarohi attempts to contact Rahul again, Vivek ignores the calls. After two months of fruitlessly attempting to contact Rahul, a broken Aarohi is forced to sing in bars again because of her family problems.
Rajveer Singh (Saif Ali Khan) has an immense passion for stock car racing and dreams of making it big on the racecourse. After being discovered by his manager Harry (Jaaved Jaffrey), he meets Radhika Shekar Rai Banerjee (Rani Mukerji) and instantly falls for her. He joins Speeding Saddles, a failing race team, and transforms from Rajveer Singh to RV the race car driver. He wins his first race and keeps winning, making him quite wealthy.
In mid-2000s, the illegal drug trade based in Kuala Lumpur is booming. The Malaysian authorities are called upon for assistance by their Indian counterparts. The Indian team sent to Kuala Lumpur is headed by Deputy Commissioner of Police D'Silva (Boman Irani), who targets the illegal drug operations of Singhania (Rajesh Khattar). Helping him along the way is Inspector Vishal Malik (Om Puri), an Indian officer from Interpol. Both of them know that in order to put an end to the illegal drug trade, they must capture "Don" (Shah Rukh Khan) who manages Singhania's business in Kuala Lumpur. Singhania was one of the two closest and most trusted men of Boris, a criminal who went underground and was presumed dead by the police. Boris's other lieutenant was a man called Vardhan, equally ruthless as Singhania, but his whereabouts are unknown. The police thinks that Singhania had killed Vardhan upon learning that Boris was about to hand over his drug business to Vardhan.