A voice-over by RD (Anil Kapoor) introduces the four main characters: Ranvir Singh “Ronny” (Saif Ali Khan) who runs a successful business he inherited from his father. His younger brother Rajiv Singh (Akshaye Khanna) who mooches off of Ronny and is an alcoholic. Ronny is dating an upcoming model Sonia (Bipasha Basu), while his personal assistant Sophia (Katrina Kaif) appears to be secretly in love with him. Ronny is also involved in an intense competition with a rival horse-owner Kabir (Dalip Tahil). The film opens with a murder plot involving a car accident which Ronny narrowly survives. When Ronny loses money in a race because his jockey had been bribed by Kabir, Ronny plants a bomb in the jockey’s car and kills him, showing Ronny to be a ruthless businessman. Later, Ronny also destroys Kabir when the latter offers him money to buy his business, which is in debt.

Sachein (Vijay), a carefree and happy going guy joins a college in Ooty and falls for Shalini (Genelia) immediately on seeing her in first sight. Shalini studies in the same college as Sachein and is considered a popular and beautiful student, while also being a short tempered and arrogant girl with an upper-middle class status. Sachein comes to know all of this information about her from Arnold aka Ayyasamy (Vadivelu), who is a guy that studies in the college for about nine years and keeps studying there because he thinks that students should give their respect to their college professors, while also informing Sachein, that there are so many boys trying to impress her. This makes Sachein take a different route and he starts mocking Shalini in front of her, which irritates her. Slowly, they both become good friends, after Sachein saved her father in a bike accident. Santhanam (N. Santhanam), who is another student in the college also loves Shalini. But, he feels jealous seeing her and Sachein being good friends and decides to separate them. Santhanam writes in one of the walls of the college, that Sachein loves Shalini and this angers Shalini, as she misunderstands that only Sachein has done it. Shalini shouts at Sachein, but Sachein informs Shalini, that he is so outspoken and does not want to write in the college wall to convey his love, which makes Shalini realize her mistake.
But to her surprise, Sachein informs that, although he has not written it, but the content present on the wall is true which means that he is in love with Shalini. This again angers Shalini and she shouts at him that she considered him only as a good friend and that she wasn’t in love with him. Later, Shalini overhears Sachein challenging Santhanam that Shalini will convey her love within the next thirty days, as their college years will be completed by then. Shalini challenges Sachein that she will never fall in love with him.
Manju (Bipasha Basu) is also from the same college and she likes Sachein. Shalini gets jealous when she spots Sachein and Manju together often. But Manju informs Shalini that Sachein loves Shalini so much and asks her not to hide her feelings, as it is very evident that Shalini too likes Sachein. This makes Shalini realize her love towards Sachein. Sachein comes to know that Shalini’s parents have arranged a wedding for her and he meets Shalini on the last day of their college and says that he has lost the challenge and he will be leaving Ooty tomorrow. But Shalini hides her feelings towards Sachein and she plans to propose her love the next day as she wants to win the challenge by not conveying her love within the 30 days.
On the 31st day, Shalini is excited and goes to propose Sachein. But suddenly, she meets Gowtham (Raghuvaran) on the way, who is Sachein’s father. Gowtham is a billionaire, in which this fact makes the entire college be surprised that Sachein is the only son of a rich business tycoon, as Sachein always stays very simple and down to earth. Now Shalini feels bad, that if she proposes her love towards Sachein at this moment, then he might misunderstand that she decided to marry him after knowing that he is so rich and decides to stay calm. Sachein leaves to the Coimbatore International Airport in Coimbatore and Shalini also follows him. In the airport, Shalini could not control her emotions and proposes to Sachein. She also revealed that it is her ego that didn’t let her convey her love during the last thirty days. Sachein feels so happy and the couple are united.

The film starts with Shakeel (Rahul Dev) killing his own brother for information, which is also known to Veeru Dada (Kolla Ashok Kumar). In the encounter, Veeru jumps off into a river from a cliff. After 18 years, we have Veeru with a leg cut off giving info to Raja (Mahesh Babu), a mischievous thief who robs banks in style. Raja gives Veeru a share of 50% for all his tip offs. Raja becomes more daring as the price tag on his head increases by thousands. There is another mischievous thief Panasa (Bipasha Basu). She and her uncle follow Raja so she can dupe him and escape with all the looted money. As the things go on a frolicking way between Panasa and Raja, Shakeel is searching for Veeru. Veeru realizes Shakeel will kill him soon. Hence, Veeru offers a big diamond to Raja and asks him to escort his daughter Bhuvana (Lisa Ray) to his brother Dharma’s place.