Munnabhai (Sanjay Dutt) is Mumbai's one of the most powerful gangsters, a man who is dreaded but is even known for his King-size heart; who's always managed to evade the clutches of law but gets encountered by Komal's (Aishwarya Rai) mesmerizing beauty and his world goes topsy-turvy. His henchmen fail to comprehend a sudden change in his demeanor, as he visualizes Komal at every point in his life, which pulled him away from his underworld activities. Finally, Munnabhai is taken to Doctor Rastogi (Amitabh Bachchan) who informs Munnabhai that he is bit by love bug and advises him to woo the girl being ignorant about the fact that it was his own sister; Munnabhai has lost his heart on.
Taking the doctor's advice, Munnabhai gets on his mission to woo Komal who is in love with Raja (Ajay Devgan), a local riff-raff who is working as a bouncer in a bowling ally. Poor Dr. Rastogi is absolutely in dark about Komal and Raja's future plans and Munnabhai's malicious intentions to break the knot even before it is tied-up. Suddenly Dr. Rastogi witnesses a complete hodge podge that makes his life miserable. To his shock, he discovers that Munnabhai's ladylove is none other than his own sister Komal. Tension mounts on Dr. Rastogi, who decides to get Komal married in U.S.A., away from the reach of Munnabhai.
Raja's love story is at stake as he is shy of his social status and wouldn't dare reveal his love for Komal to Dr. Rastogi nor could he lose Komal. However, Raja manages to go to U.S.A. along with Komal and Dr. Rastogi followed by Munnabhai! A twist in the tale comes when Raja and Komal's affair is exposed in front of Dr. Rastogi and Munnabhai's intention surfaces clearly. Raja convinces Dr. Rastogi that he would handle Munnabhai, as he owes him a favour.
A big hullabaloo rises when Raja's heroic disclosure of his love to Munnabhai brings him a dreadful warning to get off Komal's life. All efforts to get rid of Munnabhai proves futile, so Dr. Rastogi, Komal and Raja decide to stay back in U.S.A. Munnabhai plays the smart one and gets all the three back to Mumbai, where Munnabhai is an uncrowned king! What happens in Mumbai? Who wins the maze? Kaun kisse kum nahin? A rip-roaring comedy by the don of comedy, DAVID DHAWAN.