After having killed extortionist Mayil Vaaganam,[a] police officer Durai Singam covertly meets Home Minister Ramanathan on the highways, while returning to Thoothukudi. 2 days before, Durai Singam publicly resigns from police and returns to his hometown. Eight months later, he starts working as an NCC master in a Thoothukudi school, and studies the illegal activities happening, for undercover purpose. The only people who know about this operation are the Chief Minister Kumaravel and Ramanathan. Sathya is a +2 student who has heavy infatuation on Singam, though he is waiting to marry his love interest Kavya.

Singam's father Soundara Pandi is enraged about him leaving the police force. He also forbids the marriage between Singam and Kavya, as he fears that Kavya's billionaire father Mahalingam might take Singam away to manage his business empire and that Singam will lose respect with his relatives. Susai, a local who works as peon at Singam's school, shows Singam parts of Thoothukudi and tells him about the rogue happenings in those places. Singam requests Ramanathan to transfer his colleagues Erimalai and Damodarai to assist him. Singam later learns that Sathya is the niece of an influential don named Thangaraj, who is involved in Financial and Seaway Scams. Thangaraj is partners with Bhai, a criminal who controls the southeast coastline of Tamil Nadu.

Bhai and Thangaraj have connections with the international gangster and drug lord Michael Kong, aka Danny, and they are trafficking cocaine. Singam does not know about their connection and keeps Bhai in his off-duty surveillance. One day, due to a verbal fight in a pub, Bhai's chief drug importer Saghayam kidnaps a girl from another caste after her brother (another don) insults him, and holds her ransom until sunset. This causes riots between both gangs in the city, and Singam's school principal has to close the school for the day. With his info connections, Singam learns of the problem, takes charge as new DSP, rescues the girl, and returns her to her family. He is praised by the public and is also forgiven by his father.

Meanwhile, Sathya learns that Kavya and Singam are strongly in love and distances herself from them. Singam then finds out about Thangaraj and Bhai's partnership. He decides to arrest Saghayam first at the harbour, who has brought Danny with him to attend Thangaraj's new hotel party. Sagayam is arrested, along with Danny for insulting the police. Thangaraj tries to make Singam release the duo, but to no avail. Bhai sends an army of goons to bust Danny and Saghayam out of prison. Singam then finds that Danny has escaped, the police station is in ruins, and his fellow officers are injured. All of Singam's efforts are spoiled, and the next morning, he is suspended for unruly behavior. Singam warns Bhai that he knows about Danny and will return soon.

Singam meets Ramanathan at Chennai, and proposes plans for an operation to arrest all of Danny's associates in India. He then gets re-appointed and gets his superior suspended. He gets authorization to kill drug dealers on sight even without evidence. Since the next load of drugs is ready to be delivered by Danny, Bhai decides that Saghayam will deliver the drugs to him in Kerala, and he will bring it back, but he is arrested by Singam. Later, at Singam's engagement function, Mayil Vaaganam's former ally Harbour Shanmugam informs him that Thangaraj has hired goons to attack his home and family that night. Singam fights the goons himself and kills them all. He then places a drug bag insides Thangaraj's house and jails him.

The next day however, Singam gets a call and immediately goes to Sathya's home only find that Sathya has committed suicide by consuming poison, despite previously being told by Singam about her uncle's criminal activities. He analyzes that Sathya was killed by her father as plotted by Thangaraj, whom Singam confronts in prison and ensures that if he cannot prove it by arresting Danny, he will give Thangaraj a death penalty, or else Singam will kill Thangaraj and go to prison instead. Singam seeks permission from Ramanathan for traveling to South Africa as his marriage will take place in three days. Singam arrives in South Africa and joins Major Kyle Ambrose and the police to confront Danny, who has taken sanctuary with Durban-based don Alex. Singam and the Durban police give chase by car, which kills Alex in the process. Singam defeats Alex's men and confronts Danny and his men on his ship. The two fight furiously and ends with Singam's victory over Danny, arresting him, and dragging him to the same cell from which he escaped. Afterwards, he reunites with Kavya.