Plex doesn't manage your media, it just presents it and makes it accessible, if you want media managers look at Sonarr (https://github.com/Sonarr/Sonarr) and Radarr (https://github.com/Radarr/Radarr)
I haven't seen Sonarr before, it looks quite polished.
This is the first I have heard of Radarr though. (Sonarr but with Movie retrieval). I'll have to check it out.