Adventure Time (S01E05)

Episode #005
'The Enchiridion'
Season 1, Episode 5

After saving Princess Bubblegum from a nasty fall, Finn is given the secret of a book for heroes called The Enchiridion. The path to finding this book is riddled with trials and tricks, as Finn finds himself having issues with not being able to save everyone and questioning his heroics.

The Enchiridion is the first "real" adventure in the show, and for that I really enjoy this episode. Although this is still a fluff entry in the show (as you could expect from nearly everything in seasons 1 & 2), this episode does tackle a few moral issues in a fairly light way that helps establish the way the rest of the show would eventually turn. This isn't a huge moral lesson and it's never too preachy about how important it is to be "good", but it still gives the first example of a problem that Finn wasn't technically able to fix -- even if it was just a "problem" that didn't technically exist outside of trickery perpetrated by some evil fairy-gnomes. Even though this episode wasn't used just to introduce a new character, The Enchiridion itself would come into play in later episodes and becomes a huge plot point in late season 4, so yet again, this is still an establishment episode in some regard. But it doesn't just feel like a vehicle to introduce the book, but rather a full and fun adventure that brings something new to the table. Out of the first 5 episodes, this would have to be my favorite.