Sonarr is a tool that monitors for releases. Give it a show to monitor for, and it’ll pull in the air dates for each episode. When an episode airs, it looks for it on the sources you give it (rarbg etc – or usenet) and passes it to your downloader for processing. Once it’s downloaded, Sonarr can sort and file it in the relevant folder for Plex to index.
Guides to come soon – for now, refer to the sonarr wiki.