here i show how to do it using "the_content" filter - In your plugin add these codes - 

function my_plugin_post_filter($content) {
  // assuming you created a page or post entitled 'my_certain_post'
  if ($GLOBALS['post']->post_name == 'my_certain_post') {
    return var_export($GLOBALS['post'], TRUE ); // you can add your modification logic here
  }
  // else returns database content
  return $content;
}

add_filter( 'the_content', 'my_plugin_post_filter' ); // filter declaration 

For more filters, read the doc - http://codex.wordpress.org/Plugin_API/Filter_Reference

hope that helps to get idea before doing something big, If you have more questions, post
in forum in this site to get sorted out.