'Thirupatchi' begins in a remote village in Madurai where Sivagiri, a blacksmith and his sister Karupayi share a close bond. Sivagiri wishes to find a well-to-do and sophisticated groom from the city for his beloved sister.
He and his friend Kannappan succeed in establishing an alliance in Saligramam, Chennai. After the wedding, when Sivagiri comes to Chennai to leave his sister and her husband, he encounters rowdies and anti-social entities, freely engaging in criminal activities without the fear of punishment. He wonders why the city-dwellers are maintaining a mute silence to the anti-social individuals. Meanwhile, he also falls in love with Subha.
Then, in a cruel twist of fate, rowdies murder Kannappan, Sivagiri's best friend, in a tussle at a theater in Chennai. The death sparks a sequence of events that irrevocably alters the trajectory of Sivagiri's life. He vows to put an end to all criminals in Chennai and clean the city of rowdyism.