The kingdom of Ratnapuri faces harsh shortages and mounting unemployment as its people grow restless. With the king’s death leaving no heir and his only daughter Ratna missing, the royal council quietly selects Marthandan, a member of the royal family, to wear the crown. But the plan is far from smooth: the Rajaguru Vijayavarman schemes to kill Marthandan and install Pingalan, Marthandan’s cousin, as the ruler. Meanwhile, Veerabahu, faithful bodyguard of the late king, and his loyal men—including his son Bhupathi and daughter Madhana—are uneasy about Marthandan’s accession and dismayed by the suffering of Ratnapuri’s people.
Opening waves of protest spread across the land, led by Veerangan—Marthandan’s look-alike—backed by his ally Sagayam. The determined demonstrators march on the palace, calling for reforms and democracy. Veerangan is captured in Naganathapuram along with Madhana, but both are later pardoned during Marthandan’s coronation. The pair travel together to Ratnapuri, drawn together by shared ideals and a growing bond that soon blossoms into love.
As Marthandan arrives in Ratnapuri, Vijayavarman isolates him, insisting he remain apart and away from his wife Manohari until the coronation is complete. In Ratnapuri, Veerangan and Sagayam step in to quell trouble and Fate lands them in a tense clash with palace loyalties. After a thwarted attempt to poison Marthandan on the eve of the coronation, Veerangan—along with Ratnapuri’s commander-in-chief—helps save him, though Marthandan remains unconscious. To preserve the ceremony, the commander-in-chief and a minister persuade Veerangan to stand in as Marthandan for the coronation, and he agrees, driven by a sense of duty. The coronation proceeds, and Veerangan’s brief rule begins with sweeping reforms that lift the poor and challenge the entrenched elites. He conceals the truth from Manohari, who comes to see him as a noble protector, and she accepts him as her brother after learning the circumstances.
Tragedy strikes when Madhana is killed by Pingalan, fueling Veerangan’s resolve to seek justice. He learns, through Veerabahu, that both Marthandan and Ratna have been kidnapped and taken to an island named Kanni Theevu. Veerangan sails there, where he discovers Ratna; she is drawn to him, and he to her, as they share a mutual longing for safety and integrity. Pingalan and Karmegam arrive on the island with a chilling plan: Vijayavarman reveals himself as the island chief, forcibly keeping Ratna and plotting to marry her to secure his own rule and eliminate Pingalan. Ratna resists, and Pingalan tries to forcefully remove her from the picture.
Veerangan nonetheless presses forward and, with Ratna’s courage, rescues Marthandan and Karmegam, who had been captured earlier and are now reformed. The trio defeats Vijayavarman, and Veerangan completes his arc by marrying Ratna. In a decisive finale, Marthandan issues a public proclamation: Ratnapuri shall embrace democracy, marking a new era for the kingdom and its people.
