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Romeo (voice of Saif Ali Khan) is a dog who once lived in luxurious surroundings. One day his owners decide to migrate to London and he is left at the mercy of the servant of the house, who dumps him on the street. Left to fend for himself, he is soon cornered by the local gang — Guru, Interval, Hero English and a dog-wannabe cat, Mini — who tell him that this is their domain. Romeo does not know the street lingo and is at a loss for words at first, but he manages to win the gang over with the idea of setting up a business.
Together, they set up a successful dog-grooming business until Chhainu (Sanjai Mishra), the right hand of gangster-dog Charlie Anna (Javed Jaffrey), arrives to collect "hafta" (weekly protection money) in the form of bones. Romeo throws Chhainu out, and the others, terrified, go to Charlie to plead their case. Charlie threatens them with his trio of ninja dogs, whom he calls his Angels (prompting him to crack a "Charlie's Angels" joke), but Romeo tricks Charlie into allowing his friends to leave unhurt.
Romeo meets Laila (Kareena Kapoor), who is singing on a rooftop, and falls in love. To win her over, Laila tells Romeo he must dance with her in front of everyone at the "Moonlight Club" where she performs. Romeo says yes, unaware that Charlie Anna has long wanted her, and anyone who dares go near her is dealt with severely. However, Romeo braves the odds and dances with Laila to win her heart. Love blossoms, enraging Charlie, who captures and terrorizes Romeo. Thinking quickly, Romeo promises that he make Laila fall in love with Charlie.
Romeo does not intend to lose Laila, but plans to deflate Charlie's ego by having a disguised Mini pretend to be Laila and make it clear she's not interested. This works, but only exacerbates Charlie's ire, forcing Romeo to promise him a second meeting with Laila. The night of the appointment, however, Chhainu catches Romeo kissing Laila, who then learns of Romeo's deal with Charlie and angrily says she never wants to see him again. A furious Charlie and his Angels chase Romeo and the gang, capturing them. However, the three Angels are wooed by Guru, Interval and Hero English and Charlie is chased and caught by the city dog catchers. In the end, Romeo's selflessness saves Charlie Anna from the dog catcher; the two become friends, and Charlie himself brings Laila to Romeo in a train boxcar scene reminiscent of a similar, famous scene in the film Dilwale Dulhania Le Jayenge, causing Charlie Anna to remark, "Where have I seen this before?" In a dance number at the end, Romeo shows his left arm tattooed with Laila's name.