Delhi-based Sonia (Shehnaz Treasury), an air hostess, finds herself entangled in a dangerous game when she agrees to deliver a mysterious package for Vladimir Dragunsky (Kim Bodnia). Before passing through customs and airport security, Vladimir discreetly hands over the parcel to her, and she's fortunate not to be checked upon exiting. In a twist of fate, Sonia turns to her fiancée, Tashi Malhotra (Imran Khan), who is struggling with financial woes and unkempt appearances, to help her out.
Tashi, in a fit of desperation, recruits one of his roommates, Nitin (Kunal Roy Kapur), to take care of the delivery. Unfortunately, Nitin is preoccupied with his own troubles, particularly a severe stomach issue he’s been grappling with—affectionately dubbed 'Delhi Belly'—which distracts him from his tasks, as he famously plots to blackmail their landlord Manish (Paresh Ganatra) using embarrassing photos he plans to capture in a brothel.
Compounding the chaos, Nitin's misadventures lead him to a questionable lunch that aggravates his condition, causing him to forget the package entirely. In a series of errors, he requests another roommate, Arup (Vir Das), to handle the delivery, resulting in a mix-up where Nitin's stool sample inadvertently ends up being sent out instead of the actual package.
Adding to their troubles, Sonia's parents are hastening wedding plans—setting a date in just 30 days while purchasing a new flat just minutes away for the couple. Meanwhile, Tashi feels increasingly cornered by these mounting expectations. The situation escalates when a formidable gangster, Cowboy (Vijay Raaz), who was the intended recipient of the package, ruthlessly interrogates Vladimir and discovers the connection to Sonia’s three roommates.
As Tashi juggles his growing attraction to fellow journalist Menaka ([Poorna Jagannathan]), and confronts her unyielding husband Rajiv ([Rahul Singh]), complications multiply. Tashi's professional life intertwines with his personal dilemmas, culminating in a party where secrets are exposed, leading to a chaotic chase through the streets of Delhi, punctuated by gunfire.
Amidst all this, Cowboy pieces together the blunder concerning the packages and sets his sights on Tashi and his friends. Misfortune strikes as Nitin takes a turn for the worse, ending up in the hospital where he discovers the package—intact and undelivered—after a mix-up regarding its contents. When Nitin finally returns home with it, he encounters Cowboy, who recognizes the diamonds within and orders the elimination of the three roommates.
However, fate intervenes spectacularly as the apartment overhead collapses during the confrontation, eliminating Cowboy’s henchmen and leaving Tashi, Nitin, and Arup miraculously unscathed as they seize the opportunity to flee with their newfound fortune.
In a desperate bid to protect Sonia from Cowboy, who has now kidnapped her, Tashi joins forces with Menaka, leading to a frantic heist to reclaim the diamonds. However, their plans are thwarted by the police, culminating in a chaotic encounter where Tashi risks everything to save Sonia, confronting the unfortunate consequence of his kiss with Menaka.
The diamonds eventually slip through their fingers in the chaos, leading to a fallout between Tashi and Sonia when the truth of his betrayal surfaces. Nitin cunningly saves most of their funds from the police's clutches, while Tashi finds solace in a rekindled relationship with Menaka, despite his broken engagement.
In an ironic twist of fate, the diamonds, cleverly hidden inside a Russian Matryoshka doll, end up in the hands of Manish, who remains blissfully unaware of the treasure he now possesses, placing it carelessly among his home decor.
