Echarikkai starts with a backstory of David (Kishore), a 19-year-old young man and his 5-year-old nephew Thomas (Vivek Rajgopal). Thomas' father, who is also the brother-in-law of David, is a man of domestic violence who often beats his wife. One day, when Thomas' father asks money from his mother, and the latter refused, his father kills his mother, leaving David and Thomas completely shocked. Angry David kills his brother-in-law at the scene, and asks Thomas to take the responsibility for him, as Thomas is too young to be held criminally liable. Thomas agrees, but later when police arrives and slaps Thomas, he becomes scared and tells police the truth. David is therefore arrested.

After 15 years of humiliation in prison, David is finally released. During the past 15 years, Thomas, who lost both parents and uncle, was living an extremely poor and indecent life. David reunites with Thomas, and does not hate Thomas for testifying against him 15 years ago, as he understands Thomas was so young at that time and could not stand fear or pressure. The two come up with a plan to kidnap a rich people to demand ransom. They choose Swetha (Varalaxmi), the only daughter of wealthy builder Perumal (Jayakumar).

They kidnap Swetha and keep her as hostage in a remote place, and inform her father of the kidnapping. They also warn her father against involving police in the case, or they will kill his daughter. Perumal is terrified and too scared to report the case to police, instead, he seeks the help of retired police officer Nataraj (Sathyaraj), who cannot come out of his house as he needs to take care of his daughter Pia (L. Alice), who has a rare medical condition and requires constant care.

In order not to be noticed by kidnappers, Nataraj decides to solve the case unofficially and secretly. He calls his former colleagues to his houses, instead of police station, to work on the case. Meanwhile in Swetha's current place, while David leaves the house to contact Perumal, Swetha manages to fight with Thomas and takes his gun. Thomas removes his mask at the scene, revealing that not only the two knew each other already, but are actually lovers. Swetha becomes angry at Thomas for kidnapping her, while Thomas explains this is his plan to elope with her, as her father would never agree their marriage. They also plan to take away the ransom, without sharing it with David.

David demands Rs. 80 million from Perumal, in return of his daughter alive. Perumal agrees but only tells Nataraj that the ransom is Rs. 10 million, as most of his assets are illegal. Nataraj's men manage to find out identities of Thomas and David. David and Thomas also learn that police are involved. They cut a pinch of hair of Swetha, and send it to Perumal, threatening if police do not step out of the case immediately, it will be Swetha's head the next time. Perumal therefore requires Nataraj stop investigating; Nataraj agrees.

David designates a place to Perumal to put the money, while later Thomas contacts Perumal again, without David's knowledge, and tells him to put money in another place. David also manages to learn that Thomas and Swetha are lovers and planning something behind him. David talks to Swetha privately, successfully discrediting Thomas. Nataraj's doctor tells him that Pia can be cured in Singapore, but the operation would cost him Rs. 10 million. As Nataraj already knew Perumal is lying to him about the ransom amount and the place he is going to put the money, he comes up with the plan to steal the ransom money to save his daughter.

Perumal puts money at the Thomas designated place; David unknowingly releases unconscious Swetha, assuming Perumal has put money as he instructed. Thomas takes all the Rs. 80 million and escapes, but is stopped by Nataraj. Nataraj takes down Thomas, and takes around half of the ransom money from him and leaves. David arrives at the scene, and fights with Thomas. But they later reunite again after Thomas realizes that David kept supporting him even in jail. Swetha arrives at the scene and assumes being cheated by Thomas, shoots and kills him.

Nataraj is on the way to airport to take his daughter to Singapore for treatment, when his daughter experiences shivers and Nataraj burns the money to create fire and save his daughter.