An Indian military man named Ashok reaches Chennai but is soon chased down by thugs. He gets stabbed and stumbles into a church where he is discovered by Julie, Nirosha, an orphan and is admitted to the hospital. There, two men disguised as doctors kill him and before dying, Ashok scribbles IC181 on the lamp. Army Doctor Chandralekha, Srividya, who is under military house arrest for being a traitor, visits him and finds the note. Her lawyer identifies the number as a service number of an ex-military man named David, C. Arun Pandian. David and Ashok apparently worked in a secret mission involving the infiltration of an enemy prison base, and was successful, but due to the mass casualty of their team, the team was disbanded and David had reportedly quit service. Coincidentally, David and Julie are lovers. David meets Chandralekha secretly and is informed by her that Ashok was released from the military prison, and had seen her son Gunasekaran who is imprisoned there on false charges. Chandralekha requests David to rescue her son, which he agrees. He takes up the mission more out of patriotism and leaves for it.
At the same time, the hideous criminal P. K. Roy, Nassar, appoints a clever, cunning and industrious criminal Pratap, Ramki, to bring the same Gunasekaran from prison. Both leave for their missions respectively often trying to out-do each other. Pratap falls in love with a girl while he is on mission. Julie, to happen to be there for a social service program with her friends, but leave the social service program after finding out she is pregnant. She catches a glimpse of David, who rescues her from gangsters and fails to recognise her, but Julie is unable to reunite with him. With the help of a caretaker, Julie reaches Chennai.
Both Pratap and David cross paths on their mission, and when Pratap's life is in danger, David risks his own to save him and they join hands (hence the title) agreeing to work together considering the level of danger in the mission. They agree to rescue Gunasekaran together and then figure out who will get to bring him back home. An intense scene unfolds where, with prior knowledge of David and the skills of Pratap, the men rescue Gunasekaran and bring him to safety. While arguing who gets to bring back Gunasekaran, the latter explains what happened and why he was imprisoned on false accusations.
Gunasekaran is a photographer who has the privilege to take rare scenery shots at the military camps as his mother is a military doctor. At one point, while taking photographs, he encounters P. K. Roy, who supplies medicine to the Indian Army. Unknown to anyone, Roy supplies sub-standard medicines. During a shoot, Gunasekaran sees a helicopter blast in mid-air and photographs it. Roy is in the helicopter and is admitted to the military clinic and is treated by Chandralekha, and eventually loses his leg. While there, Gunasekaran tells his mother of the crash and says he is going to look for the black-box. Roy overhears this and leaves the camp without notice. Gunasekaran eventually takes the black-box and buries it safely before getting captured by the enemy military for crossing the border and is imprisoned. Chandralekha is also blacklisted for the sub-standard medicines and is arrested.
David and Pratap realize the truth and Pratap vows to protect Gunasekaran and hand him to Chandralekha. While David and Pratap are wanted by the police, Julie's pregnancy progresses. She is eventually treated by Chandralekha, but David is only able to hear her voice and learns he is going to be a father, but is unable to talk to her. He boards the train and Chandralekha is also unable to talk to him, and thus, David fails to pass the message of Julie's identity and his relationship.
As they return to Chennai, Roy arrives at the Central station too, intending to kill Pratap and Gunasekaran and retrieve the black-box. A fight ensues, and David is wounded. Pratap informs Roy of his personal vengeance as the pilot killed in the crash is actually his father. Roy eventually dies, after Pratap ties his wheelchair to a train which is leaving the station, and Roy is dragged backwards and hanged from the bridge.
Julie dies giving birth to a son, and even before David and Julie could barely hold hands, both die. Pratap takes the child and gives it to his girlfriend and tells her to take care of him till he gets released as now they will be the parents to the child of David and Pratap.
