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.

What started as an innocent love blossoms into a serious relationship as months pass. Radhika is a great pianist, majoring in music at Columbia University, whereas Rajveer has no degree or any education background. His lack of education and planning earns the disapproval of Radhika's father, Subho Shekar Rai Banerjee (Victor Banerjee). Radhika leaves her father and marries RV. She forgets about her degree and takes on her job as a wealthy housewife: Radhika Shekar Rai Banerjee turns into Shona. Their family is complete with the birth of two beautiful children, Shanaya aka Princess (Angelina Idnani) and Pradeep aka Champ (Ali Haji). Luck follows RV and he is soon proclaimed the number one race car driver in the USA.

Tragedy strikes when RV is involved in an accident whilst racing and he is hospitalised for a few months. When he gets out, he heads straight to the track to race again but soon learns that he's scared of going fast because he doesn't want to crash and get hurt again. Every race he joins, he ends up in last place. His manager fires RV and finds someone else. RV tells Harry that he doesn't need him, he can find some other manager easily.

A year has gone by and neither RV nor Shonaa have jobs. They try their best to find work but no one will hire them because they don't have degrees. RV and Shonaa are unable to pay their loans and bills so everything they own in their house is sold. Princess and Champ worry that they're becoming poor; RV says they're actually on a reality show, Don't Worry, Be Happy. With about $2,000 left, RV and family leave their big house and find a less expensive one.

They move into a one-bedroom apartment, and the children become worried again. RV and Shonaa tell them this is all part of the game and the kids believe them. They still go to their favorite school while their parents try to find jobs.

A few days later, Shonaa gets a job as a piano player and RV becomes a taxi driver.

One night when RV and Shonaa start arguing, Princess overhears them and learns that they actually are in trouble with money. She then plans to save up the money she and Champ use for lunch. The kids don't eat, and Champ gets starved. He takes food from the garbage without his sister knowing and eats it outside of school on a bench. He keeps eating from the garbage while Princess keeps saving up money. Princess makes Champ promise not to tell their parents, knowing they'll get mad.

RV gets a customer who needs to get to the airport soon. He takes this chance as to test his racing skills. RV starts off doing great until he gets flashback on his accident. He then learns that he could probably never race again because of being scared of getting hurt.

Champ gets a piece of glass inside his stomach from eating something. RV, Shona, and Princess run to the hospital where Princess tells her parents the plan she made Champ follow. The doctor tell RV and Shonaa that they need $65,000. The couple has no idea how they are going to get the money and only have two days to get it. RV decides to enter a race for his son's sake. He meets his old nemesis and he starts remembering the flashbacks. But that doesn't stop him. He wins, while apparently killing his nemesis by pushing him off the track. In the end the family is living a happy and fun life.

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.

But at the airport, Happy's and Rangeela's tickets to Australia are accidentally exchanged with that of Puneet (Ranvir Shorey) who was to fly to Egypt. In Egypt, Happy helps Sonia (Katrina Kaif) and falls deeply in love with her. But he does not express his love to her. Leaving her behind, he heads to Australia to meet Lucky. Lucky refuses to return to his hometown and throws Happy and Rangeela out of his house. Penniless, Happy finds warmth and affection in an elderly lady (Kirron Kher) who provides him with food in spite of being a stranger.

Lucky winds up in hospital, paralyzed, after a series of violent incidents that Happy causes. (In one of these incidents, his head is bumped severely, and the trauma from the resultant concussion is what paralyzes him.) Happy unexpectedly, is given the position of King. The lady who helped Happy is really worried and depressed as her daughter is returning from Egypt with her wealthy boyfriend, Puneet. The daughter does not know that after the death of her father years ago, her mother became poverty-stricken and was reduced to work as a flower seller. Happy gives her Lucky's spacious house and makes all his mafia associates work under her.

The lady's daughter arrives—and to Happy's horror, she is none other than Sonia. Happy, heart-broken, is forced to make a show of happiness to Sonia. Puneet says that he too would have had a good time with Happy and Sonia in Egypt had his ticket to Egypt not been exchanged in the airport, because of which he had ended up in Australia. Puneet had always been jealous of Sonia being with Happy. However, he doubts something going on between them.

Meanwhile, after spending so much time with the kindhearted King, most of Lucky's gang members find themselves reformed and decide to become good.

In spite of herself, Sonia falls in love with Happy—and the truth becomes difficult to hide. Puneet sets his heart to marry Sonia. Sonia gets trapped in an emotional tug-of-war between Puneet, who loves her, and Happy, whom she loves.

During the confusion, Happy's associates reveal to Puneet who they are, and that Happy is not a manager, but the King of the Australian underworld. Puneet in turn, reveals this to Sonia. Sonia becomes aware of her poverty.

Puneet meets Mika, who is also the brother of the Lucky, who agrees to kill Happy. Puneet's motives to kill Happy, however, differ from Mika's: Puneet wants to kill Happy to keep Sonia away from him, whereas Mika wants to kill Happy to make himself King.

The wedding day dawns and there is firing. Happy takes Sonia in order to save her and unknowingly both run around the fire seven times essentially getting married. (During all this, Lucky's head is bumped, and the trauma-induced paralysis he had suffered from is relieved.) Suddenly, Mika turns up on the spot, ready to kill Happy, armed with a gang and his new special glasses and hearing aid. As he is about to shoot, Lucky steps up and stops him. Then a dialogue ensues between Mika, Happy, Lucky and the associates of the kinng. Then Happy tells him that being the "king" is no great thing and explains him the characteristics of a true Sikh. Lucky confesses that he always found sadness being Kinng, because a true king does not fight for himself, but for others. Overcome with remorse, Mika drops his gun. The movie ends with Happy and Sonia's marriage.