The titular character is a dog living in Mumbai, voiced by Saif Ali Khan, and his girlfriend Laila, voiced by Kareena Kapoor. This was the first voiceover in an animated production for both actors.
Romeo 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 (voiced by 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, but Romeo tricks Charlie into allowing his friends to leave unhurt.
Romeo then meets Laila, who is singing from a rooftop, they dance and he 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 has long wanted her, and anyone who dares go near her is punished. However, Romeo braves the odds and dances with Laila to win her heart. As love blossoms between the two, Charlie, in a fit of rage, captures and terrorizes Romeo. Romeo then promises that he would 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 only ends up exacerbating 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 shouts at Romeo telling Laila of Romeo's deal with Charlie and angrily says she never wants to see him again. Charlie's Angels are then wooed by Guru, Interval and Hero English, and Charlie is chased and caught by the city dog catchers. But just before Charlie is caught with a net, Romeo pushes him under the dog-catchers van to escape while Romeo go's, Charlie then convinces Guru, Hero English, Interval and Mini to create a distraction to get the guard away from the van, they succeed and Charlie jumps on the van and says he will free Romeo using a pin in his chain to pick to lock, but the van begins to drive and he falls off, after which Charlie races after the van, losing the pin in the progress, but they improvise with one of Charlie's ear hairs. They escape but Romeo feels there no point staying as Laila said she never wanted to see him again, the next day in the morning, Romeo throws his sack onto an open train boxcar about to leave the station but Charlie arrives with Laila and the others, Charlie says to Romeo that he is a fool for leaving Laila when she still loves him, he explained everything and says she is Romeo's and no one else's, he lets go of her hand and Laila begins to run after the train, while Romeo holds his hand out for her, Charlie remarks " where have i seen this before". She reaches for his hand and grabs it but the handle Romeo was holding had lost its top screws, making him fall off with Laila, after which they raise there heads to each other and say "I love you" in sync. It then go's to the "Moonlight Club" where everyone is back singing a remix version of "Main Hoon Romeo".
- Saif Ali Khan as Romeo
- Kareena Kapoor Khan as Laila
- Javed Jaffrey as Charlie Anna, the dreaded Don of the slum.
- Vrajesh Hirjee as Guru, the oldest and wisest among the four goons. Richard Kind voices him in the English dub.
- Kiku Sharda as Hero English, one of the four goons.
- Suresh Menon as Interval, the third member of the gang.
- Tanaaz Irani as Mini, the last member of the gang of four.
- Sanjay Mishra as Chhainu, Charlie Anna's official spy.
As a fan of animation, Jugal Hansraj teamed up with Aditya Chopra to write their first cartoon. Nothing came of the collaboration, the writer/director recalls, "till this one day, I was waiting at the traffic signal and saw these dogs playing in the dirt...uncared for, scruffy looking gang, totally mast in a garbage pile, and I instantly knew the characters of my story."
The alliance by producer Yash Chopra and Disney Chairman Dick Cook is a first in the field of animation. This also is the first mainstream CGI feature film to come out of India.
Visual Computing Labs (VCL), a division of Tata Elxsi Ltd. was involved with everything from visual conceptualisation, character design to animation and final output. Production commenced in January 2007, continuing for two and a half years and involved 150 crew members; twenty-one months were spent on the animation. It also benefited from the use of Tata Elxsi's EKA, one of the world's fastest supercomputers.
Though the company did not publicly release the film's budget, Britain's Guardian reported it to be about US$7 million.
The first trailer, shown on 12 October 2007 with the release of the film Laaga Chunari Mein Daag, shows the film's crew holding an audition for Romeo; another trailer was later shown with Laila's audition. Though the release was originally scheduled for the summer of 2008, the release was put off to Diwali.
The costumes for the lead characters were made at Walt Disney World in Orlando, Florida for the film's Indian promotion. Yash Raj Films worked with Hot Muggs for producing licensed merchandise for the film. Hot Muggs integrated an inaugural concept of incorporating one-liners from the movie like "Tension Not," "Stay Cool" and "I'm Good Na" on coffee mugs.
Though many Bollywood actors promoted the film, the two main stars of Romeo, Saif Ali Khan and Kareena Kapoor did not do much promotion. Kapoor told the producers she would be busy promoting her other Diwali release, Golmaal Returns. Khan simply made himself inaccessible.