The police get information that 10 tehsildars across Maharashtra have been mysteriously kidnapped. But they are mysteriously returned – except the most corrupt officer, who is hanged in public. The police get a CD from mystery man 'Gabbar' who says his mission is to target corrupt officers. Police driver Sadhuram (Sunil Grover) is convinced that he can crack the mystery of who is Gabbar – but his superior officers humiliate and insult him in front of all the other officers.
On his way home, Professor Aditya Singh Rajput (Akshay Kumar) is stopped by lawyer Shruti (Shruti Haasan), who makes him drive her and a pregnant woman to the hospital, but the woman delivers in the car with Shruti's help. When Shruti wants to thank Aditya for helping her, she finds he has already left. Sadhuram finds from the tehsil to the district offices that there's one honest officer in each department. He is convinced that all these honest officers have a link to Gabbar. Shruti and Aditya become close and one day, she gets hit by a scooter and Aditya takes her to Patil Hospital. Although not badly injured, doctors at Patil Hospital insist on taking expensive tests and Aditya realizes how helpless patients and their families are fleeced and often, inhumanly treated. A poor woman mourns her deceased husband in the civil hospital nearby. Aditya tells her he'll get her medical insurance. He deliberately checks in her dead husband as a patient to Patil Hospital. The hospital tries to swindle money out of Aditya by pretending to treat the already dead man, making Aditya pay heavily for tests and medicines. Aditya then shows them the death certificate, which proves that they knowingly admitted a dead man. The young owner of the hospital, Vikas Patil is furious when Aditya releases the video of the hospital's corruption to the media. However, Vikas is brutally and mercilessly murdered by an angry mob. His father, a powerful builder Digvijay Patil (Suman Talwar), sees Aditya in the hospital CCTV footage and is stunned to realize that this is the same man he had supposedly killed five years ago.
Aditya and Shruti attend a wedding where Aditya remembers his late wife Sunaina (Kareena Kapoor Khan) and how they moved into a new apartment when they were expecting a baby. While Aditya was outside playing Holi, the buildings in the colony collapsed and killed hundreds of people, including his wife and unborn child. Aditya collected proof of the faulty buildings against builder Digivijay Patil, but Patil bribed everyone and no one was willing to hear Aditya out. Digvijay tried to bribe Aditya by offering Rs one crore to spare the matter but Aditya spurned the offer and beat Digvijay severely but the latter attacked him with an axe and thought his blows had killed Aditya. However, a bus full of medical students found the unconscious Aditya on a highway and save his life. He then decides to use the power of young, idealistic, honest youth and trains students at National College to join his cause. Meanwhile, Sadhuram finds out that all the honest officers attended National College for graduation. Shruti finds out Aditya as Gabbar and is shocked but when he tells her the truth, she supports his cause.
CBI officer Kuldeep Pahwa (Jaideep Ahlawat) takes charge of the case and is mystified by the facts. He listens to Sadhuram's findings and they arrest National College students. But even upon being beaten, the students do not divulge anything about Gabbar. Shruti tells Kuldeep that because the law doesn't work, a vigilante like Gabbar is a hero for ordinary citizens today. Sadhuram changes his view of Gabbar. Patil kills an honest officer to terrorize others into passing his faulty building plans. The cops are sure that Gabbar will target Patil for this outrageous act – and he does. Patil throws a birthday party for himself at his farmhouse. As he cuts the cake, he spots Aditya in the crowd. The next moment, Aditya and his gang kidnap him, all wearing similar masks. As police search the city for Patil in vain, Aditya takes him to Patil's own mansion and tells him to call anyone to save himself – but the Home Minister tells him to not involve him, so that Gabbar doesn't come after him next. Aditya and Patil then have a violent clash and he kills Patil.
Aditya goes to the police to hand himself over for the murders. He is given the death penalty but there are huge public protests and enraged students block his jail van. Officer Pahwa requests him to speak to the students and he does, telling them to channel their anger against corruption and be the change society needs. As Aditya is taken to be hanged, Shruti breaks down, but he leaves, smiling, asking the hangman, Tera kya hoga, Kaalia- quoting the famous dialogue of the character Gabbar from the film Sholay.
The songs of Gabbar Is Back are composed by Chirantan Bhatt, Yo Yo Honey Singh and Manj Musik, while the lyrics are written by Manoj Yadav, Kumaar, Sahil Kaushal, Manj Musik, Raftaar and Big Dhillon.
Bollywood Hungama rated the film 3 stars out of 5, writing that "On the whole, Gabbar Is Back is an average entertainer which will draw huge footfalls over the weekend due to catchy massy title, action drama and the fact that it is releasing on a holiday." Similarly, Koimoi rated it 3 stars out of 5, along with writing "If you enjoy films which have the good vs evil story with full on action and 'Dhamakedaar' dialogues, this is your film." Hindustan Times rated it 3 stars out of 5, stating that "Gabbar Is Back is a full on ‘masala’ film with a lot of applause worthy scenes. The buck stops at Akshay Kumar." The Times of India rated the film 3.5 stars out of 5, writing that "Now, Gabbar Is Back – and you'll feel his return rocks." Zee News also rated it 3.5 stars out of 5, writing that "After watching this we surely need a real 'Gabbar' to blow some courage in all of us—and wipe out corruption from the root. 'Pachaas Pachaas Kos Dur Jabh Koi Rishwat Leta Hai, Toh Sabh Kehte Hain Mat Le Varna, Gabbar Aa Jaaega." Subhash K. Jha of Bollywood Hungama wrote that "Akshay Gives The Super-Hero A New Definition." Hindustan Times audio news praised the screen presence of Akshay Kumar. Deccan Chronicle described the movie as "cliched but enjoyable in bits". Business Standard posted the ratings of IANS which gave 4 stars to the movie.
The film is a remake of the 2002 Tamil film Ramanaa, directed by A. R. Murugadoss, which was later remade in many other languages.