Posting front-end form to plugin controller route results in 404
You're getting a 404 because your action method isn't telling Craft what do to after it has echo'd 'Here', so routing continues.

See #3 is that link:

The request doesn’t necessarily end after a controller has been called. The controller may allow it to keep going.


You can redirect, end the request, load up a template to display, etc.

Update:

The controller folder needs to be renamed to controllers.