The kingdom of Ratnapuri is besieged by food shortages and high unemployment. Since the king's only daughter Ratna is missing and the king dies without naming a successor, Marthandan, a member of the royal family, is chosen by a majority of the imperial council of Ratnapuri to be its new ruler. However, the kingdom's Rajaguru (high priest) Vijayavarman plans to kill Marthandan and make Pingalan, Marthandan's cousin, king. Meanwhile, Veerabahu, a loyal bodyguard of the late king, and his men, including his son Bhupathi and daughter Madhana, are not pleased to see Marthandan as a king and are equally saddened by the situation of Ratnapuri's people.

Violent protests erupt all over the kingdom led by Veerangan, Marthandan's look-alike, with the support of his friend Sagayam. The protestors march to the palace condemning the monarchy and demanding democracy. Veerangan is imprisoned for his actions in Naganathapuram, a nearby town. Madhana is also incarcerated in the same prison on a similar charge. They are pardoned and released on the occasion of Marthandan's coronation. They travel together to Ratnapuri. Sharing the same ideals and revolutionary thoughts, they fall in love.

In the meantime, Marthandan arrives at Ratnapuri. Vijayavarman asks him to stay in an isolated palace on the outskirts and tells him that he should not meet his wife Manohari until the coronation, as the current moment is inauspicious. On entering a hotel in Ratnapuri, Veerangan and Sagayam beat up a gang of rowdies. Hired by Vijayavarman, they mistake Veerangan for Marthandan. Veerangan is chased by palace guards and ends up in Marthandan's room. Marthandan and Veerangan meet face to face. Veerangan tells him of the pathetic state of the people of Ratnapuri, and Marthandan agrees to set everything right as soon as he takes charge.

On the eve of the coronation, Vijayavarman arranges for Marthandan to be poisoned. He carries out his plan with the help of Karmegam (M. G. Chakrapani), a royal advisor. Marthandan swoons after drinking a few sips of the poisoned beverage, but is soon saved by Veerangan and Ratnapuri's commander-in-chief. However, Marthandan is still unconscious and incapacitated. The commander-in-chief, and one of the ministers, persuade Veerangan to take Marthandan's place to ensure that the coronation takes place as scheduled. A reluctant Veerangan agrees to do so in the interests of the state, and arrives just in time for the coronation. It takes place smoothly and Veerangan, as Marthandan, becomes Ratnapuri's new king. Vijayavarman and Pingalan are taken aback and send men to investigate. They find the unconscious Marthandan and kidnap him.

Veerangan now finds himself saddled with the role of a king for more days than he had initially bargained for because of Marthandan's sudden disappearance. He does not let this setback deter him and puts the opportunity to good use by beginning many reforms, and enacting several measures to uplift the poor. These steps are supported by the people while causing a political upheaval among the elite. However, he is troubled by the fact that he is deceiving Manohari who believes him to be Marthandan. Unable to keep it a secret from her, he tells her the truth. Perceiving his noble character, Manohari accepts him as her brother. Madhana is killed by Pingalan, and a grief-stricken Veerangan vows to avenge her death. He discovers through Veerabahu that both Marthandan and Ratna have been kidnapped and held in custody on an island named Kanni Theevu.

Veerangan sails to Kanni Theevu and comes across Ratna, who falls in love with him; he reciprocates her feelings. Pingalan and Karmegam arrive there only to find that the island's chieftain is none other than Vijayavarman, who has kidnapped Ratna after she was born. Vijayavarman plans to marry Ratna himself, get rid of Pingalan, and declare himself the ruler of Ratnapuri. Pingalan decides to take Ratna back and make himself king legally by marrying her. Ratna does not approve of it. Pingalan takes her away by force. Veerangan saves her from Pingalan's clutches. The duo then frees Marthandan and Karmegam, who had been captured by Vijayavarman and is now reformed. Both Marthandan and Veerangan defeat Vijayavarman. Veerangan marries Ratna, and Marthandan issues a proclamation stating that Ratnapuri is a democracy.