Aalavandhan is a action and thriller movie. Vijay is the commander of an anti-terrorist squad. He gets engaged to Tejaswini who is pregnant with their child. Vijay's brother Nandu is psychotic and is in a mental asylum. Vijay takes Tejaswini to the asylum so that Nandu can meet her. In his delusions, Nandu decides that she is his evil stepmother. Nandu, who killed his stepmother when he was twelve, decides that he must kill Tejaswini to protect Vijay. Nandu escapes from the asylum and begins to look for Tejaswini.