add_filter( 'template_include', 'include_template_function', 1 );

function include_template_function( $template_path ) {
    if ( get_post_type() == 'movie_reviews' ) {
        if ( is_single() ) {
            // checks if the file exists in the theme first,
            // otherwise serve the file from the plugin
            if ( $theme_file = locate_template( array ( 'single-movie_reviews.php' ) ) ) {
                $template_path = $theme_file;
            } else {
                $template_path = plugin_dir_path( __FILE__ ) . '/single-movie_reviews.php';
            }
        }
    }
    return $template_path;
}