Ishaan Nandkishore Awasthi (Darsheel Safary) is an eight-year-old boy who dislikes school and fails every test or exam. He finds all subjects difficult, and is belittled and berated by his teachers and classmates. But Ishaan's internal world is rich with wonders that he is unable to convey to others, magical lands filled with colour and animated animals. He is an artist whose talent is unrecognised.
Samar Anand (Shahrukh Khan), a major in the Indian Army, defuses a bomb in Ladakh without fear or regard for his safety. Akira Rai (Anushka Sharma), a Discovery Channel filmmaker, dives into a river in Ladakh on a dare and soon regrets it after learning she has plunged into freezing water. Samar, who happens to be by the shore, rescues her. Samar gives her his jacket and leaves without retrieving it. Later, Akira finds his diary in the jacket pocket and begins reading.
Prem (Salman Khan) lost his parents at an early age. He lives with his elder brother Rajesh (Mohnish Bahl) and uncle Kailashnath (Alok Nath). Rajesh manages the family business and his family is on the look-out for a suitable bride for him. Professor Siddharth Chaudhary (Anupam Kher) and Mrs. Chaudhary (Reema Lagoo) have two daughters, Pooja (Renuka Shahane) and Nisha (Madhuri Dixit). The Chaudhary couple and Kailashnath are old friends who meet again after several years. They arrange a marriage between Rajesh and Pooja. From their first meeting, Nisha and Prem start bickering lightheartedly with each other, and the fun and mischief continues throughout Pooja and Rajesh's wedding.
Prem (Ranbir Kapoor) is a free-loader who, while helping his friend elope, meets Jenny (Katrina Kaif). They both share the trait of developing a stammer when they are emotional. In such a moment, the stammering Jenny induces a spontaneous stammer in Prem, which she misunderstands for mockery. Although this makes her dislike Prem initially, when she finds out his stammer is authentic, the two become friends.
Bajirao Singham (Ajay Devgan) gets transferred to Mumbai and he discovers that one of his team members (Mahesh) is found dead with large bags of money. Bajirao decides to unearth the truth behind it and in this process, he collides with a powerful Baba Ji, Satyaraj Chandar (Amole Gupte) who has high profile connections with politicians.
Rahul Khanna (Shah Rukh Khan) and Anjali Sharma (Kajol) are best friends in St. Xavier's College. The college principal Mr. Malhotra (Anupam Kher) has a beautiful daughter Tina (Rani Mukerji), who comes to St. Xavier's from London to finish her degree. She becomes friends with both Rahul and Anjali. When Anjali realizes that she is in love with Rahul, she is encouraged by her motherly guardian Rifat Bi (Himani Shivpuri) to tell him so. Instead, Anjali is left heartbroken when Rahul tells her that he loves Tina. Anjali decides to leave college and shares a tearful goodbye with Rahul, going away from Rahul and Tina's lives forever. Tina then realizes that she came between two best friends. After college, Rahul marries Tina and, in a year of marriage, Tina gives birth to a baby girl. Tina has complications in her pregnancy and has little time left, so she writes eight letters to her daughter (one for each of her first eight birthdays), so that she will know her mother. Before dying, Tina asks Rahul to promise her to name their daughter Anjali.
The film opens with two cousins – Heena (Asin Thottumkal) and Bobby (Jacqueline Fernandez), the two daughters of the Kapoor family who hate each other very much – start quarrelling over something relating to their duties of Animals' Law Authority. Their fathers, Chintu (Rishi Kapoor) and Dabboo (Randhir Kapoor), are step-brothers who hate each other to the very core. Even their wives hate each other. Chintu wants the richest son-in law for his daughter, Heena, while Dabboo too wants the same for his daughter, Bobby. Aakhri Pasta (Chunky Pandey), a marriage counsellor, is ordered by both Chintu and Dabboo to find the best son-in-law. Aakhri Pasta brings Mr. Babani (Virendra Saxena) to discuss Babani's son Jai.
Lakhan Singh (Sonu Sood), a.k.a. Lucky, is the "king of the Australian underworld". He is accompanied by his Sikh mafia associates, Julie (Neha Dhupia), Mika (Jaaved Jaffrey), Pankaj Udaas (Yashpal Sharma), Raftaar (Sudhanshu Pandey), Dilbaugh Singh (Manoj Pahwa) and Guruji Gurbaksh Singh (Kamal Chopra). In a small village in Punjab, the birthplace of Lucky, lives another Sikh- Happy Singh (Akshay Kumar). Happy, though good at heart has unintentionally caused many problems in the village, and the villagers are fed up with him. They decide to send Happy on a long trip to Australia with his friend Rangeela (Om Puri) to bring Lucky back to Punjab, which will keep Happy out of the village for some time and bring peace to the village.
The film starts in the 1960 Summer Olympics in Rome, where a coach says "Bhaag Milkha Bhaag!" The story is taken back to the memories of the childhood days of a young boy which haunted him, resulting in him dropping to fourth. Partition of India in 1947 caused chaos which resulted in mass religious violence in Punjab in British India, killing the parents of Milkha Singh (Farhan Akhtar). He reaches Delhi and later meets his sister there. Living in impoverished refugee camps, Milkha soon makes friends and survives by stealing along with his friends. He falls in love with Biro (Sonam Kapoor), but she asks him to live a life of honesty.
Jagadish (Vijay), a captain in the Indian Army, returns to Mumbai from Kashmir. On his arrival, his parents and younger sisters force him to see Nisha (Kajal Aggarwal), who they choose for him to be married to. At the bride-viewing ceremony, Jagadish makes up excuses to avoid marrying her, which includes commenting about her being old-fashioned. On the contrary, Nisha is a college-level boxer, who is completely modern in her outlook. Jagadish realises this and proposes to her, which she reciprocates.
Zooni Ali Beg (Kajol) is a blind Kashmiri girl who travels without her parents for the first time with a dance to Delhi to perform in a ceremony for Republic Day. On her journey, she meets Rehan Qadri (Aamir Khan), a Casanova tour guide who flirts with her. Although her friends warn Zooni about him, she cannot resist falling in love. On her last night in Delhi, Rehan and Zooni spend the night together and end up in bed. As Zooni is leaving the next day on a train, Rehan comes in and takes her away with him. Her parents arrive in Delhi to marry them. Zooni has a procedure done that helps her see again, but when she comes out of surgery, she finds out that Rehan was killed in a bomb blast in the city, and is heartbroken. She is taken to the hospital mortuary, and told to identify his body. As Zooni cannot see any body there, she is quickly made aware of the sweater that Rehan was wearing which she knitted for him covered in blood and soot.
Abbas Ali (Abhishek Bachchan) lives in Chandni Chowk with his sister Sania (Asin Thottumkal). When their ancestral property is taken from them by deception, Abbas loses his job. Their family friend, Shastri (Asrani), convinces them to move to his village, Ranakpur, where he assures Abbas that he will get him a job at his boss, Prithviraj Raghuvanshi's (Ajay Devgn), place. Pritvhiraj is a powerful but kind-hearted man who hates lies and liars, and punishes them harshly. Abbas and Sania arrive at Ranakpur and meet Abbas's friend Ravi (Krishna Abhishek), Shastri's son, who runs a drama company with his friends. Abbas learns that there is a temple at the border of Ranakpur and Kherwada, the village where Prithviraj's cousin Vikrant Raghuvanshi is powerful. The two cousins are archenemies. The temple, being at the border of the two villages, was disputed and has been locked for years.
Jai and Meera are a modern-day couple living in London. They are happy with each other, but do not believe in tying each other down so, when career beckons, they have a mutual break-up, but decide to remain friends. Meera goes to India, while Jai stays in London, hoping to be called to San Francisco, where working at the Golden Gate Inc. is his dream job. Jai begins to date a blonde named Jo, while Meera returns the feeling of her boss, Vikram. The couple believe they have moved on.
Sameer Kapoor (Abhishek Bachchan) and Kunal Chopra (John Abraham) are fun-loving, womanizer bachelors settled in Miami. Sameer (Sam) is a nurse by profession and is often joked around by others about the cliché 'skirt wearing' joke. Kunal Chopra (Kunal) is a photographer and is in look out for apartment after his partner asks him to move out within 2 weeks. They both briefly meet each other at a friend's place after a night out with girls. Later, they run into each other while looking for an apartment to rent and they both love it. The apartment belongs to Neha Melwani (Priyanka Chopra) who lives there with her Maasi Aunty. However, Maasi refuses to sublet the apartment to either of them as Neha wanted only girls as flat-mates. Neha works for Verve magazine.
The film opens up by showing a large crowd outside the Verona Arena, Italy, with eager fans queuing for the rock concert of Jordan (Ranbir Kapoor). Jordan, who was being attacked by thugs moments ago, manages to escape and reach the arena. He shows a complete disregard for his fans, storms onto the stage in a fit of rage and starts strumming his guitar.