Manmadhan movie revoles around Two brothers look a same (Silambarasan) who is the younger one is a play boy who enjoys his leisure time partying with girls, while the older Simbu is a calm type. But he has another face. A psychotic.
Problems starts when the girls, who parties with the younger Simbu are murdered one by one.
Meanwhile a lady doctor, Mandhira Bedi has been called to treat the older Simbu who has a mental disorder. She founds out that he is the killer whom the police are looking for. This brings the twist in the story.
Why he is the killing spree, and why he got mentally upset are told in the remaining part of the story.