During 10th-century in Thanjavur, the Chola dynasty rules prosperously in South India, under the reign of Emperor Sundara Chola (Prakash Raj) (the emperor of Chola empire), whose sons Aditha Karikalan (Vikram) (the crown prince and the commander of Northern troops in Sundara Chola's Reign) and Arulmozhi Varman (Jayam Ravi) aka Ponniyin Selvan are heading their conquests of Kanchi and Lanka, respectively for the empire. Aditha has defeated the Kanchi king Khottiga (Babu Anthony) in battle but spares his life despite Kottiga wanting a glorious death in battle, and not life of insult & humiliation.Sensing something is amiss with the courtiers, Aditha Karikalan sends his friend, the Vaanar Clan's prince, Vallavaraiyan Vandiyadevan (Karthi) (the brave, adventurous and sarcastic warrior prince of Vaanar clan) to investigate and relay the news to his sister Princess Kundavai (Trisha) (also known as Ilaiya Piratti) and Sundara Chola subsequently.Vandiyadevan reaches the fortress of Kadamboor and overhears the conspiracy devised by the treasurer and minister of finance, Periya Pazhuvettaraiyar (R. Sarathkumar) (from the Paluvettaraiyar clan who was respected for his valor and 64 scars he got in battles and Chinna Pazhuvettaraiyar's elder brother) and other ministers to stop Aditha Karikalan from ascending the throne and install his uncle Madhurantaka Chola on it. They believe Sundara's (who was the younger brother to the then king, slain in battle) ascension to the throne was a mistake and that the emperor should have been from the ruling family and now is the time to correct this historical wrong. At the time, Madhurantaka (Rahman) (Uttama Chola, the foster son of Sembiyan Mahadevi and Gandaraditya Chola (the erstwhile ruler of Chola Kingdom and Sundara's elder brother)) was a child and hence not suitable for the throne. Sundara was a caretaker only, but he decided to install Aditha as crown prince and not revert the throne to MadhurantakaVandiyadevan had frequent meetings with a Vaishnavite Azhwarkadiyan Nambi (Jayaram) (a Veera Vaishnavite spy who works for the Prime Minister and Queen Mother Sembiyan Mahadevi), whom he spots at the fortress spying on the meeting. After escaping from the fortress, Vandiyadevan confronts Nambi on a boat, who reveals that he was in the fortress to meet his foster sister Nandini (Aishwarya Rai Bachchan), who mysteriously disappeared from the house during his absence and later married Periya Pazhuvettaraiyar, who is much older than her. After learning this, Vandiyedavan meets Pazhuvettaraiyar's wife Nandini (and reveals that he is a friend of Aditha Karikalan), who tells him to meet her at her building and gives her ring to him for entry into the royal palace at Thanjavur when he expresses that he wants to deliver Nambi's message to her.Vandiyedavan meets the fort's chief-in-charge Chinna Pazhuvettaraiyar (R. Parthiban) and reveals to Sundara Chola about the conspirators, but Chinna Pazhuvettaraiyar, who is Pazhuvettaraiyar's younger brother intervenes his meeting with Sundara (but not before Vandiyedavan had delivered his message to the emperor) and tries to capture Vandiyedavan, who manages to escape from them and meets Aditha Karikalan's sister Kundavai through Nandini, who orders him to bring her reply first to Nandini before reaching Aditha Karikalan. Periya reaches Thanjavur and meets Chinna and learns about Vandiyadevan delivering a message to Sundara about imminent danger to the dynasty. Chinna tells Periya that he suspects Nandini of giving his signet ring to Vandiyadevan. Periya confronts Nandini who admits that she gave the ring to the spy as she wanted to know whom he was working for and why. Vandiyadevan overhears their conversation where Nandini insists to Periya that Vandiyadevan will bring Kundavai's reply to her, before going to Aditha.Sendhan Amudhan (Ashwin Kakumanu) (a flower seller, the original son of Sembiyan Mahadevi and Gandaraditya Chola and he is a close friend of Vandiyathevan) helps Vandiyadevan to get to Kundavai's palace. Vandiyadevan confirms that Madhurantaka was present at the conspiracy meeting and agreed to the plan to dislodge Sundara from the throne. After learning about the conspiracy, Kundavai (on whom Vandiyadevan begins to develop feelings) tells him to bring Ponniyin Selvan from Lanka.Sembiyan Mahadevi (Jayachitra), the wife of Gandaraditya, foster mother of Utthama Chola/Madhurantaka and original mother of Sendhan Amudhan tells Madhurantaka that Gandaraditya gave the kingdom to Sundara and that Madhurantaka had no rights to it. She says Gandaraditya ordered him to live the life of a Shiva devotee. But Madhurantaka is adamant that he will press his claim to the throne.After winning the battle in Kanchi, Aditha Karikalan is told to return to Thanjavur but refuses as Nandini is present in Thanjavur. It is revealed that Aditha Karikalan was in love with Nandini, but the royalty refused to accept a poor orphan as the prospective queen of the Chola empire and banished her from the city. After years, Aditha Karikalan met Nandini tending to his enemy Veerapandiyan (Nasser), who was the king of Pandya dynasty who had adopted Nandini after being banished from the Chola empire. Nandini begged for Veera Pandiyan's life (who was probably her lover), but In a fit of rage, Aditha Karikalan beheaded Veera Pandiyan causing a depressed Nandini to swear vengeance against his family. Presently, Aditha Karikalan knows that Nandini married Periya Pazhuvettaraiyar just to avenge her past.After winning the war against Raja Mahinda in Lanka, Ponniyin Selvan meets Vandiyadevan, who relays messages for him from Kundavai and Vanathi (Sobhita Dhulipala) (Ponniyin Selvan's love interest. a shy Kodumbalur princess, Periya Vellar's niece, Kundavai's friend) by reaching Lanka with the help of a rowing woman Poonguzhali (Aishwarya Lekshmi) (Samuthirakumari (translated as The princess of the Ocean)). Nambi also arrives in Lanka and is revealed to be a spy of the Prime minister Aniruddha Brahmarayar (Mohan Raman).After a face-off with Nandini in Thanjavur (during a public festival), Kundavai meets Sundara Chozhar and tells him about Periya Pazhuvettaraiyar's conspiracies. Sundara refuses to believe her since Periya has been loyal to the crown for several decades. She also says that all the chieftains are meeting in secret in Periya's palace. Kundavai arrives at the chieftains meeting unannounced that says that Aditha and Arulmozhi are about to get married soon and will chose their brides from the daughters of the chieftains. This has them fighting offer their daughters as prospective matches. Later, she tells Vanathi that Arulmozhi belongs to her.Nandini tells Periya that under no circumstances should Aditha and Arulmozhi arrive in Thanjavur, as they will easily take over the fort and the kingdom. She wants only Arulmozhi inside the fort, in which case Aditha will never attack Thanjavur. The next day, Pazhuvettaraiyar and the ministers manipulate Sundara Chozhar (by saying that his own sons are plotting & competing against each other to take over the throne) to order a prison convoy to bring Ponniyin Selvan back to Thanjavur & declare Aditha as the King. Meanwhile, Kundavai is asked by Sundara Chozhar to bring Aditha Karikalan to Thanjavur, as it would not be appropriate for the crown prince to be far away from the capital. Sundara's plan is to ensure that both sons reach Thanjavur safely and then he will decide what to do.In Lanka, Ponniyin Selvan and Vandiyadevan are attacked by Pandya Aabathudavigal (a group of devout soldiers in the Pandya dynasty, who are working under Nandini's orders), which is headed by Ravi Dasan (Kishore), but are rescued by a veiled woman Oomai Rani (who had saved Ponniyin Selvan from drowning in the Ponni river at a young age, thus giving the name).Kundavai meets Aditha Karikalan and tells him to arrive back at the kingdom by arguing that her decision to have Nandini banished was right as she took that decision as a princess, but Aditha Karikalan still adamantly refuses. Meanwhile, Pandya Aabuthudavigal's member meets Lanka King Mahinda (Shyam Fernando) and receives soldiers as help from him in the group's plan to assassinate Ponniyin Selvan (Mahinda is pissed at Arulmozhi for attacking Lanka and Ravi convinces Mahinda that with soldiers he can attack the prison ship where Arulmozhi will be defenseless). Poonguzhali, realizing that the convoy is sent to apprehend Ponniyin Selvan for political reasons reveals about it to Ponniyin Selvan, Vandiyadevan, Nambi, and others. Ponniyin Selvan makes Vandiyadevan disguise himself as Ponniyin Selvan, and leaves with Poonguzhali to meet the convoy when his well-wishers prevent him from doing so, fearing threat for his life from Pazhuvettaraiyar.On the way toward shore, Ponniyin Selvan and Poonguzhali discover that the convoy has been ambushed by Pandya Aabuthudavigal, who apprehends Vandiyadevan, misinterpreting him to be Ponniyin Selvan. Ponniyin Selvan heads towards Pandya Aabuthudavigal's ship to rescue Vandiyadevan and they drown in the sea due to a storm and are consequently, presumed to be dead. The news of Ponniyin Selvan's death reaches Sundara Chozhar, Kundavai, and Aditha Karikalan who are all devastated. Aditha Karikalan swears vengeance against Nandini for his brother's death and heads towards the kingdom to destroy her. Meanwhile, Oomai Rani/Singala Naachiyaar/Mandakini Devi (Aishwarya Rai Bachchan) (who is Nandini's mother, the deaf & mute queen) , revealed to be a lookalike of Nandini, is shown swimming in the sea to find Ponniyin Selvan, to rescue him.