You would [enqueue][1] it, this is how Javascript is added to WordPress properly.


    function my_scripts_intro() {
         is_front_page(){
             wp_enqueue_script( 'introjs','path to ..intro.js' );
             wp_enqueue_style( 'introcss','path to ..intro.css' );
            }
    }
        
    add_action( 'wp_enqueue_scripts', 'my_scripts_intro' );

To determine the state of the visitor you would probably want to use [cookies][2].


  [1]: http://codex.wordpress.org/Function_Reference/wp_enqueue_script
  [2]: http://en.wikipedia.org/wiki/HTTP_cookie