How to display success message using the Contact Form plugin
You can see in the code for the plugin on GitHub that it sets a 'notice':

craft()-&gt;userSession-&gt;setNotice($settings-&gt;successFlashMessage);


As the official documentation mentions, this can later be grabbed with the following code:

{% set message = craft.session.getFlash('notice') %}


Then used as a normal message thereafter.

{{ message }}