The film commences with a flashback, depicting the childhood of Nandini and Aditha Karikalan. Brought up in a priestly household, Nandini's life intersects with Aditha's, who is training to inherit the throne, during her visit to Pazhayarai. Their eyes meet, and love blossoms at first sight, coinciding with Arulmozhi's equestrian triumph. Meanwhile, Kundavai tries to forge a friendship with Nandini, yet a playful moment shared with Aditha compels Nandini to leave the palace abruptly. Upon discovering that Nandini is an orphan, Kundavai prohibits her from entering the palace again. Heartbroken, Nandini willfully turns away from Aditha’s pleas to reconnect.
As Aditha prepares for a two-year military service, Nandini, plagued by sorrow, prays by the river where he departed. Upon his return, they finally confess their love for one another, though Kundavai expresses concern over Nandini's beauty and her non-royal origins. Despite the obstacles, Aditha Karikalan and Nandini dream of a joyous future together. Yet, their plans unravel as Kundavai and Sembiyan Mahadevi forcibly banish Nandini during Aditha's absence, shattering his heart when he discovers her exit and her refuge under Veerapandiyan.
The narrative transitions to 968 AD, revealing the Chola empire's grief over the presumed demise of Arulmozhi Varman. At the same time, the Rashtrakuta King Khottiga amasses forces to invade Tanjore, while Pandya assassins plot revenge for Veerapandiyan's demise. Sundara Chola's uncle, Madhurantakan, seeks to claim the throne against his mother's wishes, with Parvateshvar supporting his motives. Meanwhile, Nandini finds herself wed to Parvateshvar, and while appearing loyal, she covertly aids the Pandyan rebels.
Nandini cunningly influences Sundara Cholan to issue a decree allowing for Arulmozhi's capture in Lanka, leading to a tragic shipwreck amidst stormy seas. Aditha erupts in fury towards Nandini for his brother's untimely death and heads to Tanjore for answers. Parthibendran ventures to bring Arulmozhi back when the storm disrupts their journey, only to find himself imprisoned by Sundara Chozhar, who suspects him of treachery.
With the empire’s stability at stake, Vallavaraiyan Vandiyadevan and Poonguzhali, encounter Azhwarkadiyan Nambi and Sendhan Amudhan, nursing the ailing Ponniyin Selvan back to health in seclusion. As Ravidasan and the Pandya assassins scheme against Ponniyin Selvan, Vandiyadevan's clever tactics thwart their plots.
Conflict brews as Madhurantakan declares his claim to succession, uniting with the adversarial Rashtrakuta forces seeking vengeance. Through daydreams tied to Aditha's Tiger Signet ring, Nandini devises a plan to assassinate Aditha, Sundara Chozhar, and Ponniyin Selvan, seeking to eliminate any threats to her agenda. Overhearing Nandini's scheme reveals the dire implications, yet Aditha dismisses the warnings of impending doom.
The climax builds in Thanjavur, as personal histories resurface and tensions escalate. The Pandyas infiltrate the fort, leading to an all-out confrontation where friendship turns to enmity. As a tragic love story unfolds, with Nandini unable to end Aditha’s life despite their tumultuous past, his decisive act reveals a profound sacrifice borne out of love.
As chaos reigns, Vandiyadevan carries Aditha's lifeless body to the ruling lords, who unjustly accuse him of murder. The public's sorrow is palpable, with Aditha Karikalan’s death marking a pivotal moment of loss for the Chola empire. As secrets unravel, Nandini, crushed by betrayal and grief, finds herself at the mercy of her emotions, leading to a tragic demise in the river Ponni.
In the aftermath, Parthibendran assembles alliances to retaliate against Ponniyin Selvan, yet a change of heart from Madhurantakan leads him to embrace the rightful path. With the restoration of justice, the rightful heir rises, and the epic concludes on a note where Vandiyadevan and Ponniyin Selvan embark on further conquests under a unified Chola reign, heralding a new era amidst bittersweet memories.
