Naina Mathur (Rani Mukerji), an aspiring teacher with a BEd and MSc, suffers from Tourette syndrome. Her condition causes her to make uncontrollable sounds likened to hiccups. Although she has been unsuccessful for the past 5 years in landing a teaching job, she is supported in her ambitions by her mother (Supriya Pilgaonkar) and her younger brother (Hussain Dalal). It is revealed that her father (Sachin Pilgaonkar) divorced her mother several years earlier, which is why Naina's relationship with him is strained. During her childhood, she was expelled from multiple schools and would witness her parents arguing about whether she is normal or special. One day, Naina receives an offer to teach at the prestigious St. Notker's School, a job which she had applied to 5 times before. When asked why she was so persistent for the particular school, Naina explains that she herself graduated from St. Notker's and was inspired by Mr. Khan (Vikram Gokhale), a former principal. He had believed in her and announced that she would never be expelled for being different. Naina's unwavering confidence impresses the current school committee and they assign her to 9F, a new class section. It is revealed that the school was desperate to hire her as all other teachers had failed to get the class to cooperate. Naina notices that the students are unruly, misbehaved and are visibly different. Shyamlal, the school's peon (Asif Basra), discloses to her that the 9F students belong to families from a nearby slum and that they were given admission to fill the government-prescribed quota for the underprivileged.
On the first day of class, Naina's students imitate her sounds and mock her. She decides to teach them in an interactive way, determined to show resilience when dealing with the class. The students prank her with liquid nitrogen and this causes a mild explosion that shatters the classroom windows. Naina prevents the class from being expelled by taking the blame and saying that it was a badly executed chemistry example. Mr. Wadia (Neeraj Kabi), the teacher of class 9A, strongly dislikes the 9F students. His class is composed of student prefects, who are academically excellent. Naina challenges Mr. Wadia that students from 9F will earn the prefect badges as well. The school's annual Science Fair project is assigned to 9A, who are prevented from mingling with 9F by Mr. Wadia. They yearn to experience the interactive experiments that 9F gets to partake in but Mr. Wadia dismisses them as inferior to textbook learning. Meanwhile, 9F warms up to Naina and begin wholeheartedly participating in class; she teaches them to be brave in their pursuit of knowledge and helps them realize their individual strengths. Aatish (Harsh Mayar), the sole student who remains cold towards Naina, secretly sabotages 9A's project in his vengeance. This is discovered by Mr. Wadia and the Principal (Shivkumar Subramaniam) decides to expel the 9F class. Once again, Naina prevents this by promising that the class will pass their final exams, but she is unable to prevent their suspension from attending school till the exams. Dejected, she tells her students that they have shattered her attempt to help them. Aatish's classmates help him realize his mistake and he publicly expresses regret, promising Naina that the class has realized her value.
All of 9F studies diligently for their exams. Shyamlal supplies Aatish copies of the question paper, but when he presents them to his classmates, they refuse to cheat and he gives up on the idea as well. Exam results prove Naina's promise as the 9F students pass successfully, with two of the students even achieving first rank. However, the class is accused of cheating as Shyamlal confesses of supplying question papers, and the Principal decides to publicly expel them during the prefect pinning ceremony. Prior to the event, one of Mr. Wadia's students reveals that he bribed Shyamlal into supplying the wrong question papers, and he realizes that 9F did not cheat. Mr. Wadia takes to the stage and announces the he, himself, wrongfully tried to fail the students. He then applauds Naina's transformative teaching and asks her to pin the prefect badge to her students which ranked first. She later tells him that she doubts that he bribed Shyamlal. Mr. Wadia smiles and says that he doubts she caused a liquid nitrogen explosion. With the combined efforts of 9A and 9F, the Science Fair project is rebuilt and wins the competition. 25 years later, Naina is shown to be the Principal, and she narrates that she is finally retiring from teaching. As she walks out of the school, the school's students praise her and bid farewell. The last to happily bid her are the students of 9F, who are now successful adults.