In very rare cases, one might need to override the default template (although it's a bit dirty to do that).
In the functions.php file, add the following code:
function wpa_overload_template( $template ) { if( isset( $_GET['parameter'] ) ) { $template = locate_template('template-overload.php', false ); } return $template; } add_filter( 'template_include', 'wpa_overload_template' );