The movie opens with a new recruit police constable Arivazhagan (Dileepan) reporting for duty at Tiruverumbur police station, in Tiruchirappalli. He is taken under wing by Head constable Thangaraj (Samuthirakani), who works as a writer at the station. Thangaraj is shown to be a bigamist, and it is later revealed that he was forced into the second marriage with a much younger girl, to bear an offspring. Thangaraj had initiated and is still continuing to fight a case, for the establishment of a police union, for voicing the opinions of lower rank police officers. This does not go well with the Inspector of the police (Bose Venkat), who has him transferred to Thiruvallikeni police station in Chennai, as a punishment.

Once there, Thangaraj is assigned menial duties within the station and befriends an ex-convict, Raja, whom Thangaraj had known from a previous case. Raja does odd jobs and is shown to stay within the station premises. On his first day, Thangaraj is assigned to watch over a PhD student Devakumar (Hari Krishnan), held in illegal custody in a marriage hall nearby. Devakumar tries to escape initially, but gets caught by Thangaraj and the manager of the marriage hall. The police personnel in the station don't seem to take any interest in revealing the details of the case. The police inspector Perumal and another head constable Manickam, force Devakumar to sign some documents.

Devakumar is constantly shifted to various lodges, and the Deputy Commissioner Trivedi Sharma is also shown to be involved in the case. Raja and Thangaraj go to watch over Devakumar, and he sobbingly shares his story to Raja. Devakumar belongs to a backward community, living with his elder brother Xavier (Subramaniam Siva), his sister-in-law and their son, in Thirukkattupalli town, near Trichy. After a spat in the house, he heads back to Chennai. Pretty soon, he is shown to be detained by the police, who also take his cellphone away. A senior journalist in Trichy, whom Thangaraj has been pursuing to write about the police union case, publishes this illegal detention as a scoop, after Thangaraj unintentionally reveals it to him.

Trivedi Sharma becomes livid, and has Thangaraj write a crime scene to implicate Devakumar, not knowing it was Thangaraj who leaked the story. It also shown that Thangaraj was previously acquainted with him. Devakumar gets implicated as a Naxalite and is arrested. Thangaraj feels guilty for having caused this. Xavier comes to Chennai to meet his brother. Thangaraj explains to him the seriousness of the situation and asks him to approach a lawyer Marudhamuthu (G. M. Sundar), to help out in the case. Marudhamuthu learns that the police are very secretive about the case and is not allowed an audience with Devakumar.

Trivedi Sharma, Inspector Perumal and head constable Manickam ask Devakumar why he wanted 'those details' and beat him up. Marudhamuthu learns of this and rushes back to the police station with Xavier and manages to convince Perumal to allow Xavier to meet his brother. Thangaraj reveals to Marudhamuthu about his role in Devakumar's arrest. He secretly smuggles a phone into Devakumar's cell to allow him to speak with Marudhamuthu. It is then shown that Devakumar is doing PhD in Sociology, from Madras University. He was researching about mental health of police officers and increasing numbers of death by suicide, in the police force.

He had contacted a lawyer Anwar, in Saidapet court, who filed an RTI to get him the information for his thesis. In a flashback, it is revealed that Trivedi Sharma had rejected three applications for the joining the horse riding team of Tamil Nadu Police force, one of which belongs to Saranya (Ineya). She is told by an officer that she was rejected because of her caste certificate. When Saranya questions this, she is forced to do stable maintenance duty by Sharma. One day, when Sharma visits the stables, she adamantly rides a horse in front of him, which angers him and he beats her up badly, and she commits suicide soon after.

Thangaraj becomes remorseful after hearing this and plots along with Arivazhagan to help Devakumar escape and surrender in a court, under a different case, thus escaping the police officers who want him silenced. After facing many hurdles, Thangaraj helps Devakumar run away. A few months later, Thangaraj is shown to be in jail. In a letter to Arivazhagan, he reveals what happened after he helped Devakumar run away. Thangaraj is hit on the head by one of his pursuers and is hospitalized, Sharma, Perumal and Manickam trace and abduct Devakumar. A guilt-ridden Thangaraj is forced to shoot him dead. Later, when Sharma, Perumal and Manickam try and convince Thangaraj to finish the case, writing it off as an encounter of a criminal trying to escape, Thangaraj refuses to cooperate and says he will reveal the truth, thereby leading to the implication of the other three. When Sharma threatens to shoot him, Thangaraj snatches the gun and shoots Sharma dead, in front of a shocked Perumal and Manickam. Thangaraj asks forgiveness from Xavier for causing his brother's death. After reading the letter, Arivazhagan is shown to continue fighting the police union case, heeding Thangaraj's request.