Aalavandhan is a action and thriller movie. Vijay (Kamal Haasan) is the commander of an anti-terrorist squad. He gets engaged to Tejaswini (Raveena Tandon) who is pregnant with their child. Vijay's brother Nandu (also played by Haasan) 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. In the process, he encounters drugs and kills other people under their influence. The rest of the movie deals with the encounters between Nandu and Vijay, who is determined to save his fiancee even if it means that he has to kill his brother.