In the 10th century, Emperor Parantaka II of the Chola Empire reigns prosperously in South India while his sons Aditha Karikalan and Arulmozhi Varman aka Ponniyin Selvan are heading their conquests respectively for the empire at Kanchi and Lanka. After winning the battle against Rashtrakutas, Aditha Karikalan dispatches his trusted associate and the Vaanar clan's prince Vallavaraiyan Vandiyadevan to go to Kadambur and investigate and subsequently pass on the information to his sister Princess Kundavai and Sundara Chozhar as he sensed something was amiss with his courtiers. Vandiyadevan encounters a Vaishnavite Azhwarkkadiyan Nambi before arriving at the fortress of Kadambur, where all the ministers have gathered. Vandiyadevan finds his way into the fortress using his companionship with Kadambur's prince Kandhamaran and overhears a conspiracy devised by the minister of finance and treasurer Periya Pazhuvettaraiyar and other ministers in a meeting to resist Aditha Karikalan from ascending the throne and install his uncle Madhuranthaka Chola on it.
Vandiyadevan witnesses Nambi spying over the conference and after exiting from the fortress, he confronts Nambi on a boat making him disclose that he was there to find his foster sister Nandini, who mysteriously disappeared from the house in his absence and later, married a much older Periya Pazhuvettaraiyar. Vandiyadevan later interrupts a convoy of Nandini and informs the latter that he is Nambi's acquaintance and would like to deliver her a message from him, intending to gain access to the palace of royalty in Thanjavur bypassing Periya Pazhuvettaraiyar's younger brother Chinna Pazhuvettaraiyar. Nandini gives him a ring that would help him enter the palace and leaves; Chinna Pazhuvettaraiyar, the fort's chief-in-charge permits Vandiyadevan as the latter presents the ring and claims that it was given to him by Periya Pazhuvettaraiyar. Vandiyadevan meets Sundara Chozhar and discloses to him about the conspirators but Chinna Pazhuvettaraiyar intervenes and attempts to have him apprehended. Vandiyadevan evades and out of misinterpretation, he is taken to Nandini's chamber where she discovers his motives for meeting Kundavai with Aditha Karikalan's message and asks him to bring the reply to her first. He obliges and is shown a secret tunnel out of the palace.
Vandiyadevan meets Kundavai in Pazhayarai through a friend and flower seller Sendhan Amudhan and relays the information about the scheme against the royalty to her. She issues an order to him to bring Ponniyin Selvan back from Lanka and he complies. Madhuranthaka's mother Sembiyan Madevi asks Madhuranthaka to desist plotting against Sundara Chozhar and his heirs but is rebuffed. Aditha Karikalan narrates to his companion Parthibendran Pallavan that he was indulged in an affair with Nandini as a teenager but she was not accepted to be the prospective queen of the empire and banished from the city. Aditha Karikalan happened to meet her after several years, tending to his enemy Veerapandiyan, Nandini's father figure. Infuriated, he beheaded Veerapandiyan and presently, emphasizes that Nandini married Periya Pazhuvettaraiyar to take revenge against the Chola Empire.
Ponniyin Selvan emerges victorious in a battle against Raja Mahinda of Lanka while Vandiyadevan engages a rowing woman Poonguzhali (Samuthirakumari) to take him to Lanka and discovers that she admires Ponniyin Selvan and harbors feelings for him. Divulged to be a spy of Sundara Chozhar's well-wisher Prime minister Aniruddha Brahmarayar, Nambi arrives in Lanka and chances upon Vandiyadevan. While Vandiyadevan transmits messages for Ponniyin Selvan from Kundavai and Vaanathi (Princess of Kodumbalur) who is expected to marry him, Nambi suggests that it is dangerous for Ponniyin Selvan to return. Kundavai returns to Thanjavur while Periya Pazhuvettaraiyar and other ministers manipulate Sundara Chozhar into dispatching a convoy to bring Ponniyin Selvan back to Thanjavur as a prisoner for rejecting the throne of Ashoka's daughter, offered to him by Buddhist monks. Sundara Chozhar commands Kundavai to convince Aditha Karikalan and return with him to Thanjavur as it would be inappropriate for the crown prince to be away from the capital.
In Lanka, meanwhile, Pandya Aabathudavigal, a group of soldiers headed by Ravi Dasan, devoted to Pandya dynasty and avenging the death of their ruler Veerapandiyan, attack Nambi, Ponniyin Selvan, and Vandiyadevan intending to assassinate the former. However, they are resisted by Oomai Rani, a veiled woman; Ponniyin Selvan claims that she was the one who rescued him from drowning in the river Ponni when he was a child. Kundavai and Vaanathi make it to Kanchi and Kundavai keeps persuading Aditha Karikalan to return and states that her decision of having Nandini banished from the city was for the dynasty's good but he refuses to agree and doesn't oblige to her requests. Meanwhile, a member of Pandya Aabathudavigal reaches Mahinda and gains from him, an army to accomplish the group's plan of assassinating Ponniyin Selvan. Poonguzhali chances upon the convoy, which is assigned on arresting Ponniyin Selvan, and realizing that it was sent by Pazhuvettaraiyar, she senses a conspiracy and refuses to disclose Ponniyin Selvan's details. She reveals the convoy to Ponniyin Selvan, Vandiyadevan, Nambi, Pallavan (who was sent by Aditha Karikalan to Ponniyin Selvan), and Periya Velar, Vaanathi's uncle.
His well-wishers repudiate sending him to the convoy but deciding to abide by his father's orders, Ponniyin Selvan seeks the help of Poonguzhali to meet the convoy and disguises himself as a mahout, while making Vandiyadevan look like a prince. Poonguzhali and Ponniyin Selvan discover that the convoy is ambushed by Pandya Aabathudavigal, who seizes Vandiyadevan, misunderstanding him to be Ponniyin Selvan. Intending to save Vandiyadevan, Ponniyin Selvan heads to the sailing ship of Pandya Aabathudavigal and fights the latter but the ship eventually drowns in the sea due to a storm while a helpless Poonguzhali tries to rescue them. Ponniyin Selvan and Vandiyadevan are presumed to be dead and the news is relayed to Sundara Chozhar, Kundavai, and Aditha Karikalan who are all devastated. Vowing to retaliate against his brother's apparent death, Aditha Karikalan sets out to Thanjavur to end the deviously deceptive Nandhini. Meanwhile, Oomai Rani, disclosed to be a lookalike of Nandini, deeply dives into the sea and intends to protect Ponniyin Selvan.