Lakhan Singh (Sonu Sood), also called Lucky, is the kinng of the Australian underworld, and being a sikh, is the first Singh Is Kinng of the film's title. 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 man more notorious than Lucky himself—Happy Singh (Akshay Kumar). Happy has caused many problems and disastrous situations 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 a man named Puneet (Ranvir Shorey) who was hoping to use them 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 sikh 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 receives the title of Kinng—thereby becoming the other Singh Is Kinng of the film's title. Alas, Happy is not proud of his rank and is kind-hearted. 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 the flower-seller 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 anything is going on between them. He and Sonia squabble about this topic; it is Happy who handles it.
Meanwhile, after spending so much time with the kindhearted Happy, 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. Sonia becomes aware of her poverty. 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.
Puneet meets Mika, who is also the brother of the Kinng, and 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 Kinng.
The wedding day dawns and there is firing (all because Julie drops some guns). Puneet hides while the pandit chants the mantras, his head totally covered by a pot, amidst gunfire all around. Meanwhile Happy takes Sonia's hand 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 (sic). 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 revolver. The movie ends happily, with Happy and Sonia's marriage.