Twin brothers, Vasu and Vel, get separated at birth and grow up in different families. One becomes an officer while the other turns into a rough character.

Govindan (Manivannan) is a thief who has two children. Aadhi (Pasupathy) and Mathi (‘Chiyaan’ Vikram). Years go by and the two, Aadhi and Madhi, decide to stop stealing and mend their ways and lead a hardworking life along with their father. They migrate to neighboring village and meet a retired agricultural officer, Chidambaram (Vijaya Kumar) who is in deep debts and is under pressure from the village’s landlord, Kalingarayar (Murali) to clear his debts. In efforts to help Chidambaran, Mathi confronts Seetha Lakshmi (Asin), Kalingarayar’s daughter, who comes to collect the money Chidambaram owes her father. Seetha Lakshmi starts to acquire a liking for Mathi but keeps it hidden due to her father’s atrocious temper. In an attempt to teach Kalingarayar a lesson, Mathi forcibly ties the mangalsutram around Seetha’s neck. Kalingarayar, realizing his daughter’s love for Mathi, comes down to arrange a grand re-marriage between the two. But things go awry when Manicka Vel (Biju Menon), Seetha Lakshmi’s maternal uncle comes to town in an effort to stop the wedding between the two as he has plans of marrying her and wiping her family fortune.

Aalwar (Ajith Kumar) is devoted to his mother (Geetha) and sister (Shwetha Bandekar). But the villainous elements Lal and Vincent Asokan kill the sister and mother due to an old dispute with Aalwar. Aalwar, with revenge ringing in his mind, ends up as a killer, even while working as a ward boy in a hospital. He rechristens himself as Shiva and is out to make a statement against the venal forces. He sees himself as some kind of avatar. In fact, he bumps off the baddies under the getup of Lord Rama and Lord Krishna. In the climax, Shiva turns up as Lord Narasimha and bumps off the last villain by placing him on his thighs and ripping apart his bowels and chest with his sharp claws. He successfully escapes from the police of Chennai and continues his process of “Dharma”.

The movie is all about a group of five college friends — Shaam (Shaam), Imaan (Arya), Pooja (Laila), Priya (Asin) and Irene (Pooja). The friends gather for Imaan’s wedding, years after everyone has parted ways.
The movie starts with Pooja leaving USA to go to Imaan’s wedding. Meanwhile, back in India, everyone else is helping Imaan with the preparations for his wedding and catching up with their long lost college friends. The movie goes through series of flashbacks of old memories during their college days. Imaan and Irene meet for the first time, after a long time. The first flashback occurs. They were once in love; however it didn’t turn as Imman had expected it to be.

Thamizh is a killer for hire with no ties and who will do any dirty job for money. He has a girlfriend who loves him but also detests his violent nature. But is Thamizh really what he purports to be?

For Kumaran (Jayam Ravi), life is about his mother Mahalakshmi (Nadhiya), who was separated from her husband Eeshwar (Prakash Raj). Eeshwar goes on to pursue his passion in kickboxing and becomes a world-renowned trainer based in Malaysia. Mahalakshmi stays in India and devotes her life to bringing up her son. Kumaran is also passionate about kickboxing and shares a special relationship with his mother. In between, he falls in love with Malabar aka Mythili (Asin Thottumkal), but his life is destroyed after his mother’s death. On her deathbed, she tells him to meet Eeshwar, and Kumaran goes to Malaysia to meet him. Kumaran gets a job at his father’s kickboxing academy but finds him with another wife (Aishwarya) and a daughter. Kumaran feels betrayed and angry, and this causes plenty of friction between him and his father. Eeshwar trains a boxer Anand (Subbaraju), whom he is sure will win the championship. Anand is Eeshwar’s devotion, and on many occasions, he demonstrates his preference for Anand over Kumaran. Anand makes Eeshwar’s daughter pregnant and dumps her. He turns traitor to Eeshwar and joins another team when they offer him a better contract.

Muthappa runs away from his hometown because of his brother. Years later , he returns to his hometown and found out that his brother ruined his family. He decides to take revenge.

Shivshankar, a person who uses a wheelchair, tries to make his son, Vishnu, aware of his responsibilities but to no avail. Things take a turn for the worse after Vishnu tries to rape a girl.

Chitra (Nayantara), a medical student and her friends are working on a project about the human brain. She wants to investigate the curious case of Sanjay Ramaswamy (Suriya), a (former) notable Chennai based businessman, who is reported to have anterograde amnesia. Her professor denies access to Sanjay’s records as it is currently under criminal investigation. Chitra, nonetheless, decides to investigate the matter herself.
Sanjay is introduced as he brutally murders a man. He takes an instant picture of the man, and writes on it “done”. Sanjay has anterograde amnesia; he loses his memory every 15 minutes. Sanjay uses a system of photographs, notes, and tattoos on his body to recover his memory after each cycle.

Bio-scientist Govindarajan Ramaswamy speaks at the Jawaharlal Nehru Stadium about chaos theory and the butterfly effect. He begins by reciting events from the 12th century in Chidambaram, when King Kulothunga Chola II, a Saivite, persecutes Vaishnavites and intends to destroy an idol of Govindaraja. Rangarajan Nambi, a Vaishnavite, protects it and offends the king, who orders Rangarajan to be executed by being pierced, stoned, submerged into the sea with the idol, before being devoured by the shark, thus the king succeeding.
On 20 December 2004, a nano bio-technology lab in the United States designs a vector-virus intended as a bio-weapon. When they were watching the insound of his lab monkey breaking out of his cage and retrieving a sample of the test virus. The monkey, who thinks the vial as the usual chocolate Govind would feed him, swallows the sample of the virus. Govind and his co-workers watch helplessly as monkey dies. Govind, saddened by the event, quarantines the lab room by filling it with concentrated salt solution, the only antidote to the virus. After understanding the original virus’s lethal potential the hard way, Govind refuses to hand over the main single vial containing the virus, due to fear of misuse. However, his boss, Dr. Sethu, has a malicious plan in mind, and tries to sell it to a terrorist group. Govind tries to sneak the vial out of the lab in order to save the vial from Sethu’s evil plan, but is pursued by the security guard and the police. Govind flees to his friend and coworker Suresh’s house for refuge, who secretly betrays him.