This project is currently abandoned, as I have switched my setup from Plex to Jellyfin, a fully FOSS fork of Emby server (already leagues ahead of its old master).
I am however developing an equivalent client for that platform.
Disclaimer: the client is a side project and learning experience for me. No guarantee whatsoever is offered about it. The target OS is Linux only.
To avoid adding a further dependency and given it is currently employed only for a minor use-case, the source file from dkjson is directly included in the repo. This is likely a temporary measure.
luarocks install --server=http://luarocks.org/dev pmcli. You will then need to manually make sure mpv is installed.
Alternatively, there is an AUR package which will install everything.
You can launch the client as simply pmcli.
On first start, you will be prompted for configuration and login.
IMPORTANT: only login with Plex credentials is supported. If you use Plex via OAuth with your Google or Facebook account, you will have to resort to manual configuration.
Navigation should be straightforward; mind that you can express command ranges as n1-n2, command sequences as s1,s2, open everything in the current menu with * and can always submit q to quit immediately.
If you encounter problems, check the troubleshooting page.