Gunaa, Kamal Haasan is an inmate in an asylum whose cellmate spins a tale about a woman named Abhirami. The story settles in Gunaa’s mind as a vivid, almost sacred vision: he believes Abhirami is a divine avatar who will marry him on a full moon day. After a battery of psychiatric tests, his psychiatrist, Girish Karnad Ganesh, approves his release, and Gunaa sets out to find the woman who haunts his dreams. Yet the real world soon collides with his conviction, and Gunaa must navigate a web of family history, crime, and faith to hold onto his fantasy.
Manonmani, S. Varalakshmi, Gunaa’s mother, runs a brothel after a cruel turn of fate left her husband. Her brother-in-law, known as “Chitappa,” acts as her aide and partner in a rough, precarious livelihood. Rosy, Rekha, one of Manonmani’s workers, loves Gunaa and longs to marry him, but his heart is tethered to the dream of Abhirami rather than to her. Gunaa himself has a knack for breaking locks, a skill he often lends to Chitappa’s schemes, and the two men move through a world of petty theft and risk with a shared, stubborn determination.
Driven by a plan to repair Manonmani’s finances, Chitappa decides to break into a temple’s hundi to steal money that could cover overdue rent. He drags Gunaa along for a rehearsal of the heist, and in a moment of chance Gunaa spots a wealthy heiress at the temple. The heiress’s form and grace seem to align with his imagined Abhirami, and when she smiles and offers him sweets, Gunaa is convinced that destiny has delivered his goddess in the flesh.
The day of the break-in arrives. Gunaa unlocks the safe room and frees Chitappa’s men to enter. He pursues the heiress again, and in the ensuing chaos, Chitappa’s crew becomes tangled in the escape. Officials intervene, yet the fugitives manage to flee with jewels, hijacking the heiress’s car in a desperate bid to outrun the law. A car crash kills one of the gang members; the wounded survivor grabs the heiress’s car and Gunaa clambers in beside him.
The wreck ends with the car plunging into the sea, and the heiress’s disappearance triggers a police investigation led by CBI officer Ramaiah. Ramaiah, the investigator, learns of the heiress’s disappearance and informs Suresh Kumar (SK), her guardian. Gunaa, who has somehow saved the heiress and the jewels from drowning, brings them to his home, where a chain of new dangers begins to form. Landlord Ismail spots the newspaper reward for the heiress’s safe return and mobilizes his goons to seize the fortune, misreading the situation as a fresh opportunity to extort Manonmani’s brood. To protect the heiress, Gunaa drives her to a dilapidated church perched on a hill, where he continues to address her as Abhirami.
Rohini, the heiress’s true name, resists Gunaa at first, hating his odd behavior and his fixation on the vision of Abhirami. With Chitappa’s help, SK and Ismail reach the church and coerce Rohini into signing blank papers to give her wealth to others, a move meant to strip her of autonomy and control. She agrees to avoid being killed, but SK remains intent on removing her. Gunaa intervenes, killing Ismail and knocking SK unconscious, then carries the injured Rohini to a cave for safety. Over time, Rohini begins to understand the depth of Gunaa’s love. She reveals her real name but chooses to keep the name Abhirami as a symbol of the love she now sees in him, and she accepts Gunaa’s marriage proposal. They both agree to marry on the night of a full moon.
The morning after their wedding, Rohini falls ill with fever. A doctor is summoned to treat her, and the news of Gunaa’s involvement in the affair reaches Inspector Moovendhar. In a tense sequence, Gunaa steals a gun and escorts the doctor to the cave as Moovendhar rallies his force. A standoff unfolds with the police, and Gunaa ends up firing a shot that hits a policeman before retreating to the cave with the doctor to tend Rohini. Moovendhar escalates his pursuit, issuing a shoot-at-sight order against Gunaa.
Aided by Gunaa’s kin—Ganesh the doctor, Manonmani, Rosy, and Ramaiah—there is a public appeal to avoid killing Gunaa. Rosy ventures into the cave to retrieve the gun, carrying it back to Gunaa to keep his enemies at bay while they attempt a peaceful surrender. Rohini steps forward and reports SK’s crimes, a revelation that enrages SK, who shoots Rohini in a bid to silence her.
Enraged and heartbreak-stricken, Gunaa kills SK and returns to the cave, where Rohini dies in his arms. In a moment of raw, tragic clarity, Gunaa tells his family that humans cannot truly grasp the depth of his devotion to Abhirami, and he walks away from the world, carrying Rohini’s body with him as the night of the full moon finally rises.
