Plex Documentation → Your Media → Naming & Organizing TV Shows
Adventure Time, correct?
Make sure you use Plex recommended naming and structure.
Plex Dance the show if you have to rename/reorganize.
/TV Shows <-- folder added to TV Show library. Name does not matter
.../Adventure Time (2010) {tvdb-152831} <-- Show_Name (year), TVDB/TMDB ID optional.
....../Season 01 <-- "Season" in English. Two digits for number
........./Adventure Time (2010) s01e01.ext <-- sXXeYY mandatory. ext = mkv/mp4/etc