Gunaa is an asylum inmate. His cellmate tells him a story, whose protagonist Abhirami gets registered in Gunaa's mind; he believes and dreams that she is an avatar of goddess Abhirami who would marry him on a full moon day. After his psychiatrist Ganesh sanctions his release, Gunaa keeps searching for Abhirami. His mother Manonmani runs a brothel after his father deserted her. Her brother-in-law, addressed as "Chitappa", is her aide. Rosy, one of the brothel girls, loves and desires to marry Gunaa, but he does not reciprocate.

Gunaa is an expert at breaking open locks, and Chitappa regularly takes his assistance for stealing. Chitappa decides to break open a hundi at a nearby temple to pay Manonmani's overdue rent. When he takes Gunaa for a rehearsal, Gunaa spots a wealthy heiress in the temple. Since her physical attributes match those of Abhirami as imagined by Gunaa, he approaches her and she smilingly gives him some sweets; her pleasant disposition convinces him that she is Abhirami.

On the scheduled day of heist, Chitappa, Gunaa and their men arrive at the temple. Gunaa opens the lock of the safe room and lets Chitappa's men inside. Seeing the heiress again, he goes after her, accidentally locking Chitappa's men inside. They are caught by officials but manage to escape with some jewels and flee with Gunaa via car. One of them dies when the car crashes, the wounded survivor seizes the heiress's car and starts driving it, with Gunaa also joining.

The wounded man succumbs, and the car falls into the sea. Ramaiah, a CBI officer, investigates the heiress's disappearance, and informs her guardian Suresh Kumar (SK). Having saved the heiress and the jewels from drowning, Gunaa takes them to his house. Landlord Ismail sees the financial reward in a newspaper for the safe return of the heiress. His goons assume her to be a new prostitute brought in without paying their usual bribe and fight with Gunaa. To save her, Gunaa drives her to a dilapidated church on a hilltop, and keeps addressing her as "Abhirami".

The heiress hates Gunaa for his weird nature and tries to escape, but fails every time. With Chitappa's help, SK and Ismail reach the church; SK coerces the heiress to sign blank papers so he can control all her wealth. She assents to avoid being killed, but SK tries to kill her anyway. Gunaa intervenes, kills Ismail, knocks SK out and takes the injured heiress to a cave for safety. Though she first hated Gunaa, she now understands his deep love. She reveals her real name is Rohini, but prefers the name Abhirami and accepts Gunaa's marriage proposal. Though Gunaa is adamant on marrying on a full moon day, Rohini convinces him it is that day and marries him.

The next morning, Gunaa realises that Rohini has contracted fever; he approaches a doctor to treat her. The doctor's assistant, having seen Gunaa on a newspaper, informs Inspector Moovendhar. When Moovendhar arrives, Gunaa steals a gun and forcibly takes the doctor to the cave. Near the spot, both men are surrounded by numerous policemen led by Moovendhar; Gunaa accidentally shoots a policeman before retreating to the cave where the doctor treats Rohini. Moovendhar seeks more forces and issues a shoot-at-sight order against Gunaa.

Gunaa's kin including Ganesh, Manonmani, Rosy and Ramaiah reach the spot and tell Moovendhar not to kill Gunaa. Rosy enters the cave and requests Gunaa to return the gun which he does through Rosy while remaining inside. Moovendhar then orders Gunaa to surrender. Rohini informs Moovendhar that they are fine, coming out and decides to report SK's crimes. SK overhears this and, to hide his crimes, shoots Rohini. Infuriated, Gunaa kills him. When Gunaa returns, Rohini succumbs in his arms. Gunaa then angrily tells his kin that humans cannot understand his love for Abhirami, and jumps off the cliff with Rohini's corpse, coincidentally on a full moon day.