Plot
In 1979, a 17 km pipeline was installed to transport crude oil, crossing 40 villages and connecting Chennai Harbour and Ennore, with the OPC crude oil storage unit. The natives resisted, fearing it would harm their livelihoods, but the government used its authority by killing two organizers, effectively ending the protests. Manoharan, one of the organizers, secretly tapped the pipeline and began illicitly smuggling crude oil, using the money to benefit the community without revealing his identity. Years later, Manoharan's adopted son, Vasudevan "Diesel Vasu", a chemical engineer, expanded the illicit trade by transporting it to Pathaan's Mumbai refinery, with bribed government officials turning a blind eye. The refined petrol and diesel were then mixed with the legal supply and distributed in Chennai. Vasu further expanded the operation by disguising white petrol as ice cubes to evade police detection and channeled the money for welfare activities through his friend, Sedrayan "Jettu".
DCP Mayavel, a ruthless officer, takes bribes from Manoharan, but often clashes with Vasu. Balamurugan, with Mayavel's backing, starts stealing from lorries by bribing drivers, adulterating fuel with coloured water. The adulterated petrol damages vehicle engines, prompting police to investigate the supplying pumps. Vasu discovers Balamurugan's involvement and warns lorry owners. Balamurugan retaliates, using political influence to send a councillor to threaten Vasu, but Vasu resists. A meeting is arranged between Manoharan, Mayavel, Balamurugan, and oil syndicate members. Mayavel threatens to halt Manoharan's dealings unless Balamurugan's business is allowed, challenging anyone to stop Balamurugan from tapping crude oil like Manoharan. Manoharan's oil tankers are confiscated, angering Vasu, who steals Mayavel's jeep to threaten him, despite Manoharan's warnings to stay calm. Mayavel faces heat from superiors and, enraged, attacks Vasu, but Vasu retaliates, severely injuring Mayavel.
Police hunt Vasu, and Manoharan sends Vasu to his friend, Pottu Sekar's custody, but Sekar's wife, Kayal, opposes sheltering Vasu. Vasu and his friend Murthy hide in a mid-ocean boat. Malar, a law student, tests her dream of a mermaid saving her by going into the ocean. When her boat stops for repairs, she falls in, and Vasu rescues her. She learns about Vasu's biological parents, Dillibabu and Neelam, and is surprised when Neelam appears as a mermaid in her dreams. Malar confesses her love for Vasu. Mayavel recovers and starts his killing spree, humiliating and arresting Manoharan, and sealing his petrol pump. Vasu remains unaware in the ocean, while Balamurugan clears Vasu's slum settlements and takes control of the oil syndicate, bribing and threatening members. Following Jettu's warning about the police, Vasu moves to Kakinada port after sending Malar and friends back to land. Balamurugan brutally kills Jettu and excessively taps around 2 crore liters of oil, prompting a probe under Rudraksh Bhatia, Chief Secretary. Mayavel exploits fishermen's hatred towards Manoharan, learning Vasu's whereabouts, but Vasu and Murthy narrowly escape Mayavel's check and reach Chennai.
Vasu learns of Jettu's death and that they unknowingly traveled with his corpse the previous night. He also discovers Manoharan's arrest and Pathaan's mastermind behind it. Earlier, Pathaan had stated that by bribing the government, he had kept the pipeline open, aiming to build a private Chennai port, with plans to vacate the 5,000-acre area occupied by slum residents. Manoharan severs his ties with Pathaan, while Manoharan had asked not to disclose his arrest to Vasu, fearing for Vasu's safety if he returns to Chennai. Manoharan's lawyer, Namachivayam, refuses help, angered by Malar's love for Vasu. Pottu Sekar motivates Vasu to find the missing 2 crore liters of crude oil to expose Pathaan's misdeeds. Rudraksh Bhatia and his team begin their probe, while Vasu apprehends corrupt oil transport in-charge Moorthy, learning Pathaan's plan: moving into battery technology, using surplus Lithium from Jammu and Kashmir, aiming to lead global battery manufacturing. So, Pathaan needs a harbour for business ease, so he looted crude oil, hoarding it secretly, to make people switch to his batteries.
Malar secures Lawyer Srinivasan to bail Manoharan out. Mayavel's plans to murder Manoharan in court are thwarted by Vasu after receiving information from Malar. Mayavel learns that Balamurugan double-crossed him and met Pathaan, and so he humiliates Balamurugan in front of his wife, Mahi. Vasu deduces that the stolen crude oil is stored in an unused nearby water pipeline and shares this with the slum people. The slum people gain trust in Vasu, realizing his past welfare funding through Jettu. Vasu makes the slum women protest by asking for water supply through the idle pipeline by blocking the highways, and makes the slum men block ship entries into Chennai harbour with their boats. Vasu offers Rudraksh Bhatia help retrieve the crude oil and begins to reveal Balamurugan's oil smuggling activities. Fuel scarcity increases, causing concerns, inflation, and daily activities grind to a halt. Pathaan contacts the Union Home Minister to stop the fishermen's protests, and the Union Minister orders Rudraksh Bhatia to halt the probe. Angered, he demands that Vasu reveal the oil's location. In return, Vasu demands that the government stop Pathaan's private harbour plan and remove the crude oil pipeline.
Rudraksh Bhatia has Vasu share his demands with the media, showcasing a viral video highlighting the slum's miseries due to the pipeline and effluents. The video sparks public outrage, temporarily suspending Pathaan's private harbour project and triggering inspections at his refinery. Meanwhile, Manoharan is bailed out, and the Chief Minister orders negotiation talks with the protesting slum women. Manoharan's aid to Mahi's toddler reaching the hospital prompts Mahi to rebuke Mayavel's revenge mode. Realizing Balamurugan's manipulation, Mayavel orders him to release the hoarded crude oil, but Pathaan refuses, ordering the control or destruction of the oil and protesters. Vasu has already secretly transferred it via barrels with Bhoominathan's help, moving it to fishermen's boats in the harbour. Police attack the protesting women, and goons target Manoharan simultaneously. Mayavel and Vasu intervene, stopping the brutality, but Bhoominathan is killed in the shootout. Manoharan is killed by Balamurugan, avenging his father's death.
Enraged by the missing crude oil, Balamurugan orders Vasu's death. Pathaan's oil industries are permanently shut down for illicit hoarding, and his harbour is temporarily banned. Vasu is punished with a 2-year sentence for illicit trade and released. The court orders the OPC oil plant and pipeline's permanent removal, restoring slum and fishing activities after 40 years. The film ends with Vasu revealing he killed Balamurugan and his men before serving his prison term, avenging Manoharan's death.
