Chidambaram, a kind-hearted simpleton from a wealthy family in Karaikudi, has cherished affection for his cousin Uma since childhood. Uma, Ilavarasi, now a well-educated woman, has fallen for Arun, her college friend. When Chidambaram's parents, Kaathamuthu Chettiyar, [Kishmu], and Valayapatti Valliammai, [Manorama], propose marriage to Uma's mother, Ponnazhagi, [Ganthimathi], she ridicules Chidambaram for his naivety.
Determined to prove himself, Chidambaram accepts a job with Ambalavaanan, a temple trustee who secretly runs a diamond-smuggling racket. Ambalavaanan, [Sangili Murugan], hires him, and he unknowingly transports diamonds, which lands him in the hands of the Aravaiyal police. After a daring escape, he hides in the car boot of burglars who have raided a house. The burglars tie up the household and flee, and Chidambaram, mistaken for a member of their gang, stumbles upon a murdered dancer, Aasha, [Disco Shanti], leaving his fingerprints on a dagger.
Back home the next morning, newspapers tie him to diamond smuggling, burglary, and murder, deepening the suspicion around him. Uma's father, Nachiappan, goes to the police to report his son-in-law-to-be missing but learns that Chidambaram is now a wanted man. To clear his name, Uma enlists Arun, [C. Arun Pandian], a college friend, to help bail him out and prove his innocence. The police, wary of a single man being accused of three crimes, decide to bring in CID officer Beemarao, [Visu], who is assigned to the case with the benefit of doubt.
Beemarao's arrival is met with an assassination attempt when goons attack him, barely thwarted by Arun's timely intervention. Beemarao is invited to stay at Masilamani's grand house—Masilamani, [Delhi Ganesh]—as he pursues the trail. A second near-fatal incident follows when a ceiling fan falls in Beemarao's room, hinting at a broader conspiracy. Convinced of Chidambaram's innocence, Beemarao starts to follow a lead that points toward Ambalavaanan's circle, visiting Ambalavaanan's house and noticing his peculiar habit: drawing coconut trees while talking on the phone.
Back at Masilamani's residence, Beemarao, Arun, and Uma discuss a possible ruse to verify the theory that Ambalavaanan staged a theft report. They retrace Chidambaram's arrest sequence, visiting a petrol pump where a torn page in a telephone directory is found—another page bearing Ambalavaanan's signature coconut-tree doodle. In a twist, Masilamani is revealed as the mastermind behind the smuggling operation, the infamous 'Black Cat', and had been listening in on Beemarao's conversations. His plan to warn the thieves is foiled by Beemarao's investigation and cunning.
Beemarao's detective work continues at Aasha's house, where the culprit's habit of drinking lemon tea is uncovered. The next morning, Beemarao notices Masilamani performing the same ritual, providing a crucial clue that seals his guilt. To trap Masilamani, Beemarao stages a fake arrest of the dacoity gang. Masilamani, alarmed, plots to abduct Chidambaram and the gang en route to court. Beemarao assigns Arun to protect Chidambaram, where he fights off Ambalavaanan and his men, ultimately arresting him.
In a dramatic reversal, Masilamani discovers that the supposed dacoity gang members are actually police inspectors in disguise. Arun handcuffs his father, and the police offer him a fresh start in service. Chidambaram is acquitted of all charges, and at Uma's marriage to Arun, Chidambaram, though heartbroken, selflessly helps Uma and Arun with the wedding preparations.
