Somu (Gemini Ganesan) is the only eyewitness of his mothers murder. The face of the killer is clearly imbibed in his mind. Years later as a youth he leaves home on a mission which is to find his mother killer and take revenge. His younger brother Ramu (MGR) is an honest Police officer and is in love with Jaya (Jayalalitha) daughter of a rich entrapraner Doraiswamy (M.N. Nambiar). The views of both the brothers vary. Somu wants revenge by killing the killer while Ramu says that the killer should be handed over to the law. Will Ramu be able to convince Somu or will Somu succeed in killing the culprit. The answers to these questions lie in Mugaraasi.