The film starts with Dhivakar and his family celebrating his birthday, where a sudden lightning strike kills him and his son and burns them to death. At a movie theatre, a man gropes a woman, and a small commotion ensues, which was stopped by ACP Paul Dhanarajan, who later dies in the middle of a police conference. The plot shifts to detective Kaniyan Poongundran, who lives with his associate Manohar. They both come across Mallika through a pickpocketing incident where she gets caught. Later, she comes to work as a housekeeper at Kaniyan's house.

One day, a schoolboy named Naveen, visits Kaniyan and Manohar with the case that his pet German Spitz (inaccurately identified as a Pomeranian), Nemo was shot dead and asked them to find the killer. Naveen gives them the bullet from Nemo's body. Kaniyan finds that it is a 9mm bullet which had been ricocheted from Nemo. At the crime scene, he finds a tooth, whose owner is traced to be Swarnavel, who was formerly a university physics professor who divorced his wife because he blamed her for the death of their son. Most recently, he was living alone and working as a stockbroker. Meanwhile, a shady man named Kamalesh visits a travel agency to deliver a sofa stashed with money to five people who run the agency and who are believed to be assassins: Devil, Muthu, Pritha, a stout man, and a bald man.

Kaniyan recollects a news article about an incident that involves the deaths of Dhivakar and his son and goes to Mrs. Dhivakar's house, where she reveals that thunder occurred in the beginning, followed by lightning, which struck Dhivakar and their son. Kaniyan also finds that Swarnavel was present at a nearby construction site on the day of the incident. He then concludes that it was artificial lightning devised by Swarnavel to kill Dhivakar. When the assassins learn that Kaniyan is on their tail, Devil disposes of the body of Swarnavel, who was killed earlier. Kaniyan and Manohar visit Dhivakar's office and learn that a tender which was to be awarded to Dhivakar is now with Ram Prasad, the chairman and head of the company that Dhivakar competed with.

Before they could reach Ram Prasad, he is assassinated. Infuriated that their client Ram had to be killed, Kamalesh hires Chinese mercenaries to kill Kaniyan under the pretext of inviting him and Manohar to a Chinese restaurant. Kaniyan defeats them and finds out about Kamalesh's involvement, but Kamalesh is seduced and killed by Pritha before they arrive (but Pritha is seen by Kaniyan while leaving). A police chase occurs as Pritha and the bald man try to escape but are later cornered from all sides. Soon after, the bald man kills himself by performing seppuku, allowing Pritha an opportunity to escape the scene. The next day in a shopping mall, Pritha makes a failed attempt to kill Kaniyan, who is saved by Mallika.

Kaniyan and Manohar meet Police Inspector Madhivanan and ACP Vijayakumar in a hotel, where yet another failed assassin attempt is made. Muthu plans to inject Kaniyan with a poison dart, but Manohar was caught in the crossfire and is rushed to the hospital. They learn that Manohar was hit by the same dart as Paul. Kaniyan goes to the theatre to confirm this and gets to know that on the same day, three tickets were booked by the same person but in different rows. He gets the address from the credit card details. They raid the travel agency, and Devil kills Muthu to cover his tracks. They find some physics books related to lightning as well as a machine that generates artificial lighting.

Kaniyan receives a threat via SMS, saying that his beloved ones are in danger. He ensures that Manohar is safe in the hospital, but could not save Mallika in time as she was stabbed by Devil. She pickpockets his mobile when he stabs her, through which Kaniyan gets to know about the Inspector who was keeping Devil informed about all the moves and status of the investigation. It is later revealed that Madhivanan was an informer for Devil. Kaniyan, Manohar, and a police team go in search of Devil to a mangrove forest. All of them die except Kaniyan, Manohar, and Vijayakumar. Later, Kaniyan and Vijayakumar are both caught by Devil, as he blackmails them using Naveen (who was also kidnapped by Devil).

Tied by a rope, Kaniyan reveals how he managed to uncover Devil's backstory and tells him that the police have been informed of the same. Infuriated by this, Devil tries to stab Kaniyan, but Manohar manages to untie him just in time. In the ensuing scuffle, Kaniyan gains the upper hand despite their more or less equal size and skill in hand-to-hand combat; eventually, Kaniyan kills Devil. In his dying moments, Devil tells Naveen that he is sorry for killing Nemo. Later, as Naveen returns home from school one afternoon, he finds a Pomeranian puppy along with a note bearing Kaniyan's name in a basket at his doorstep.