How can I configure CKEditor to show content in 'plain text editor'?
Assuming Drupal 7 - goto admin/config/content/formats and change the order of the text formats, placing Plain Text at the top of the list. This will default the edit format, however content that is already created would default to whatever format was used to create it in the first place.

This will however filter quite restrictively the output. If you mean to include a text format that does not perform any formatting at all, on the same page, you can 'Add Text format', label it unaltered or whatever title you prefer, set roles to administrator (very high security risk with giving this format to untrusted persons), and leave everything else blank. This will create an unaltered format which allows things like direct html, javacript and php writing in blocks, etc.