Ability to enter maintenance lockdown mode: force-logout all non-administrators, prevent logins from non-administrators, and disable commenting.
Nothing goes in the plugins directory.
pmc_lockdown.php to the
/wp-content/mu-plugins/ directory. Simply placing this file activates it. To deactivate it, move it from that location.
The user interface for turning the "Maintenance Lockdown" mode on and off is in the Settings | Privacy section of your site's adminstration system.