Ramanaa begins with the abduction of 15 Tahsildars. After three days in captivity, 14 officers are released, but one is murdered. The police find a tape recording and a file on the dead officer's body that provide significant evidence concerning the kidnapping and subsequent murder. The file suggests that the crimes were committed by members of the self-proclaimed "Anti-Corruption Force" (ACF), a citizen militia seeking to eliminate corruption through vigilante acts. The militia, acting as a kangaroo court, "sentenced" corrupt officials in the area to death, beginning with the captured revenue officers who were apparently engaging in criminal activity.
The film's setting then shifts to M. Ramanaa, a mild-mannered professor at National College, who lives in a house with four adopted kids. While treating his adopted child at a big private hospital, Ramanaa discovers that the officials at the facility are engaging in extortion and fraud. After collecting evidence of the hospital staff's deception, Ramanaa turns them over to the authorities. The ringleader is the hospital dean, Rishi, who commits suicide when this is discovered. His grief-stricken father, a big and powerful construction magnate named Bhadrinarayanan, vows to take revenge on the man responsible for his son's death. Meanwhile, the ACF goes on kidnapping the top 15 corrupt officials each month and then killing the #1. Those officials include officials from transport, PWD, revenue, sports, health, etc. Then, suddenly, Devaki, a college girl and friend of Ramana's kids, finds out about him being the leader of the ACF. She questions him about this immoral activity.
Seven years ago, Ramana was a college professor with a small cute family, of him, his nine-month pregnant wife Chithra, and his daughter. During the festival of Deepavali, Ramanaa's apartment building collapses due to heavy construction work near-by, killing many residents including Chitra and his daughter. Recovering from the incident, he demands to know who was responsible for the building's collapse. He discovers that the building was built and owned by Bhadrinarayanan, who knowingly constructed the complex on loose soil. When Ramanaa confronts the District Council with evidence, Bhadrinarayanan walks in and boasts about all the bribe he has thrown to the top district officials including the collector. he gives Ramanaa a chance to call any two top officials in the state and get him arrested, but both the officials who are called immediately hang up. Frustrated, Ramanaa seals the room and attacks all officers, and as he is about to kill Bhadrinarayanan, he is beaten badly by his men, and is thrown on a highway. He is then rescued by National college students, who were on a trip. In response to Bhadrinarayanan's actions, and the general corruption in the region, Ramanaa forms the Anti-Corruption Force (ACF).
Meanwhile, a local police constable, frustrated for being not promoted, as he could not bribe officials, begins secretly building the case against the ACF. The constable eventually realizes that the ACF is primarily composed of people who do not take bribes. He visits multiple revenue offices, but his superiors neglect him due to him being their low associate. Then, a message comes saying that civil supply officers are going to be taken. All corrupt officials try to get police protection by proving their corruption, but at the last moment, the ACF switches to Police department. All corrupt civil supply officers are arrested, and the government promises to employ 25,000 youngsters within a week. 15 district police chiefs are kidnapped. The police officer is killed, and everyone is frustrated. An IPS Punjabi officer is flown from Delhi to head the case.
Meanwhile, Bhadrinarayanan is trying to re-corrupt the officials. As the final stroke, the top 15 dons are kidnapped. Ramanaa personally comes to Bhadrinarayanan. As was done to him, he gives the same lifeline to Bhadri, to call any two people within India. This time, Bhadrinarayanan fails, and is kidnapped and later killed. Then, the constable reveals his plan to his superiors, and the IPS officer is delighted.
The officer takes swift action against them. The ACF members are captured and tortured by the police but refuse to reveal Ramanaa's identity. Ramanaa is overcome with guilt over his students' sacrifice and surrenders himself, on the condition that all the other members of the ACF be released. He is tried and convicted of being the mastermind organiser behind the murders of Bhadrinarayanan and 14 other government officials, and is sentenced to death. Public pressure from the sympathetic community and family makes the Chief Minister offer Ramanaa a pardon appeal, which will definitely give him complete freedom. However, Ramanaa refuses, stating that only he must face the consequences of his actions, and is hanged the next day. The constable is offered a promotion for his role in disbanding the ACF, but he refuses out of guilt.