Police officers Adhi Narayanan and Abbas devise a plan to take down a terrorist group; they send undercover officers Anand and Shiva on "Operation Dhanush". The objective is to infiltrate a terrorist group headed by the Naxalite Badri and relay information back to Abbas. After a year of the operation, Anand is caught and commits suicide before he can be interrogated. With the information relayed by Anand, Abbas and Adhi know the terrorists are picking up someone important and are going to Tiruchirappalli.

Abbas tightens security at the railway station while Adhi calls his wife from a public telephone booth. Adhi sees some suspicious activity and tries to investigate but a sound from his radio alerts the terrorist and a shootout follows. Adhi wounds a terrorist and Adhi stops another from fleeing by shooting at the car's tyre. The wounded terrorist enters the railway station but is shot dead by Abbas and the car driver is taken into custody.

Adhi asks the driver for information on Badri. The driver maintains his innocence but Adhi is convinced the driver holds a mid-level position in the terrorist organisation. The terrorists plan to assassinate a central-government minister who is visiting the city. A rocket-propelled grenade (RPG) expert is brought in to kill the minister, whose route the terrorists seem to know. The terrorists kill the minister but Adhi catches the RPG shooter, who identifies the driver as Badri.

Adhi tells Badri the RPG shooter has identified him and is willing to give more information about the terrorist organisation. Badri uses his influence and kills the jailed RPG shooter. Enraged, Adhi beats Badri and decides to kill him but instead interrogates him. Badri reveals the terrorists have planted a spy in the police department; Srinivasan – Abbas and Adhi's mentor. Adhi tells the Central Bureau of Investigation (CBI) Srinivasan is a spy but Srinivasan commits suicide before he can be arrested.

Badri's assistant Narasimhan sends terrorists to kill Adhi's pet and shoots Adhi's son to show he can easily kill Adhi's family, who Narasimhan threatens to kill if Badri is not allowed to escape. Adhi cooperates, lets Badri escape, and becomes the terrorists' new spy. Abbas becomes suspicious and follows Adhi when he goes to meet Badri, who catches, tortures, and kills Abbas.

Griefstricken, Abbas's wife Zeenath and their teenage daughter go to live with Adhi, his wife Sumitra, and their son. Badri again threatens Adhi to allow two terrorists, Surendar and Mala, to live with and monitor Adhi. Overcome with guilt and grief over Abbas's death and his betrayal of his duty, Adhi realises he is becoming like Srinivasan and decides to end the arrangement. Adhi sets up a meeting with Badri. Shiva, who is still in contact with Adhi, tells him Narasimhan is meeting his wife and that if Narasimhan dies, Shiva will become Badri's assistant. Adhi kills Narasimhan.

Adhi tries to attack Badri in a safe house but is overpowered. Surendar tries to rape Abbas's daughter and kills Mala when she intervenes. Sumitra intervenes and seduces Surendar, steals his gun, and kills him.

Adhi is tortured for information about Dhanush. When only Adhi, Badri, and Badri's second-in-command Shiva are left in the room, Badri tells Adhi the group has found the radio used by the spy and that Dhanush – the spy's codename – can no longer contact the police. This surprises Adhi; he looks at Shiva, who is the spy, for confirmation. Badri notices this and realises Shiva is Dhanush. When Badri tries to kill Shiva, Adhi breaks free, wrestles with Badri, and kills him.

Adhi, deciding the mission to destroy the terrorist organisation is more important than his life, orders Shiva to shoot him so Shiva can continue spying. Shiva shoots Adhi as the other terrorists enter the room. Shiva tells the terrorists Badri was killed by Adhi and that he killed Adhi. Shiva says Badri's death leaves him, as second-in-command, to take over the group. Adhi and Abbas are felicitated posthumously.