‘Jolly LLB’ is a social satire on the law system of the country, where a small town lawyer Jolly (Arshad Warsi) is pitted against a hugely successful lawyer, played by Boman Irani. The film begins with a drunk Rahul Dewan (Rahul Siddhartha), racing in an Sports Utility Vehicle against a car, after he is seduced by a woman in that car. He loses control and crashes into a wall on the footpath by the road.

The film cuts to a court scene where Jagdish Tyagi aka Jolly (Arshad Warsi), an LLB graduate defending a surgeon against a PIL filed against him after the surgeon had his son operate upon a patient in his pursuit of guinness book of world records. Jolly loses the case but decides to move to Delhi to try his luck. He happens to see prominent criminal lawyer Tejinder Rajpal (Boman Irani) defending Rahul Dewan in a session’s court. Rahul Dewan is accused of causing death under the influence of alcohol. Rajpal convinces the judge that prosecution had no evidence to implicate Rahul Dewan and he was being deliberately targeted by the media as he was from a rich family, resulting in Dewan's acquittal. Rajpal, though, is not satisfied with the fee paid by the Dewans.

Jolly, desperate to have some money and fame, decides to pursue the case and files a PIL in the court against Rahul Dewan's acquittal. After initially reprimanding Jolly for his mistakes in filing the PIL and taking the press reports as evidence, Justice Tripathi (Saurabh Shukla) gives a date for hearing and warns Jolly to collect some evidence before the hearing. He then runs into Albert Pinto (Harsh Chhaya), who claims to have witnessed the accident. Then Jolly introduces him as a witness in the court and Judge Tripathi, after cross-examining him, gives a date for the next hearing. He also summons Rahul Dewan to be present in the court for the hearing. Jolly becomes a celebrity overnight and is admired by Kaul who donates a room in his restaurant to Jolly for using it as his office. However, Jolly's plans turn upside down, when Albert Pinto reveals to him that he is no witness, but a crony of Rajpal and it was part of a plan to extract more money from Dewan clan. With the help of his Brother In-law Pratap (Manoj Pahwa), he collects the video footage of the SUV involved in the accident and presents it to the court. Though Rajpal counters him that the SUV was driven by the driver of Dewan family, Jolly refutes his statement by submitting the necessary evidence.

Judge Tripathi orders the Police to provide a bodyguard for Jolly after he was manhandled by alleged assistants of Rajpal. Jolly comes to know that a survivor of the accident stays in Gorakhpur and decides to leave for Gorakhpur. SI Rathi tries to sabotage Jolly's investigation, but is rescued by his bodyguard. He identifies one Sadakanth Mishra as the survivor in Gorakhpur and brings him to Delhi. Jolly prosecutes SI Rathi and finds that he has botched up the investigation by declaring Sadakanth Mishra as dead and snatching his hard-earned money to stop him from killing Mishra. Mishra also identifies Rahul Dewan as the one who drove the SUV.

After the conclusion of arguments and counter-arguments between Rajpal and Jolly, Judge Tripathi directs Delhi Police to suspend SI Rathi and initiate a criminal inquiry against him and also declares Rahul Dewan as guilty of the crime and sentenced him to seven years in jail. The film ends with Jolly celebrating with his team while Rajpal is crestfallen as he suffers defeat after a long time