Kaadhal Kondein deals with the story of a youth who is mentally and physically abused as a child. Vinod (Dhanush) has grown up in a church where the Father has looked after him. He has always lost people he loved the most. Throughout his childhood the absence of his mother had troubled him. As he grows up he is compelled to attend college for further studies. But he is a complete misfit there. He finds a friend in Divya (Sonia) although the rest of the class ostracizes him. Vinod soon falls in love with Divya but he also feels that Divya does not consider him any more than just a friend. He also comes to know that Divya is in love with Aadhi (Sudip).
On the other hand Divya's father is furious with her relationship with Aadhi. He locks her up in a room. Vinod, however, arrives at her house and succeeds to meet her on the pretext of borrowing some old clothes for himself. Taking pity on his condition Divya's father allows him to meet her. Vinod utilizes this opportunity to free Divya from her confinement. He also triumphs in convincing her that she will be able to meet Aadhi as well at Ooty.
Vinod, however, has completely different plans for Divya. He has planned to win her heart over once they are in Ooty. Vinod takes Divya to a secret place and make s her stay with him. In the meantime he keeps on convincing her that Aadhi would definitely come. During their stay Divya comes to know about the Vinod's unhappy past. He narrates the story of how he worked as a paid labourer and how he revolted against the oppressors. He was severely beaten for his audacity. He also loses his girlfriend as she is killed by her rapists. He barely managed to escape from the inferno and took shelter in the church with the Father. Divya is moved to tears by such horrendous string of events.
In the meantime while Vinod is away to get food the police and Aadhi arrive at the place and explain to Divya that Vinod is a psychopath. But Divya does not believe in their arguments. She rather defends Vinod, explaining to them his decent behaviour. On the other hand as Vinod learns about the police he is furious. He resorts to violence by firing and a police constable. He hoodwinks them and takes Divya away with him. This time he professes his love for her. Divya politely turns him down saying she can only be a friend with him.
Aadhi finds out both of them and prepares to attack Vinod. They fight horribly and at the end Vinod, Aadhi and Divya find themselves hanging at the edge of a cliff. While Divya hangs on to tree bark the men manage to hold either of her hands. Divya is pushed in to a situation where she has to choose between her boyfriend and friend. She does not have the heart to hurt Vinod. The cliffhanger scene finally ends with Vinod giving up his life for her. He lets her hand go, falls into the deep valley and dies.
The film was a runaway success for its unique storyline. The story also led the p[ath for the making of films like Chellamae and Kudaikkul Mazhai. But they could match the magic of the original version.