Police Commissioner Suryadeo Singh (Amrish Puri) is a ruthless law enforcer whose very name strikes terror in the hearts of criminals. A firm believer in the motto "To eliminate crime, eliminate the criminal" the super cop has only one weakness - his grand daughter Kajal (Twinkle Khanna) who he loves more than his own life... she is his "Jaan". Unfortunately Suryadeo Singh has many enemies and Kajal is now in acute danger. A man who can face death in the eye is absolutely shaken; he cannot even trust the police force to protect his "Jaan". After a thorough investigation, Suryadeo Singh discovers a trained weapon expert Karan (Ajay Devgan) who he deems fit to protect Kajal. Destiny intervenes and Kajal falls desperately in love with Karan, even as Karan hopelessly tried to stay aloof from her. But no matter how much he denies his true feelings, Kajal had indeed become his "Jaan". But how can he reciprocate her love? The truth haunts him - the terrible truth that he was planted by Suryadeo Singh's enemies and is doomed to murder his own love. "Jaan" is the clash between two ruthless men and a young innocent girl torn between her grandfather and her love for an assassin.