Tip: if your video files are already named after a common pattern (ideally, already after the Plex norm Show Title - SxxEyy - Episode Title), you can use mp3tag’s “Convert: Filename → Tag” feature to write the episode title into all files at once.
The format string %dummy% - %dummy% - %title% might do it for the above pattern.
(Please ignore the mkv file name extension in this screenshot. Tagging these mkv files would unfortunately do nothing in Plex. I only used these files to illustrate the process in mp3tag.)
