Episodes like 'Holly Jolly Secrets' (Part II, specifically) are exactly why I love "Adventure Time" (well, one of the many reasons). There's more going on here than a show like this would let on, and for a kid's show, there's uncommon depth. What started out as a discovery of secrets in Ice King's diary sheds just a sliver of light on his origins. It's not a long sequence, and the opening of the episode certainly doesn't give you any indication there'd be heavy stuff coming. I don't want to spoil anything here, but the Ice King wasn't sad and creepy years before, and seeing the transformation carries more than a little bit of sadness.
But it's a Christmas episode, and they bring you right back from the sadness with a moment of levity before our characters take up real estate by the fire, "to celebrate the day when Finn and Jake had a fleeting moment of empathy for the biggest weirdo in Ooo".
And what's really cool about this backstory is that it hints at a larger story for the Candy Kingdom.
Shockingly good stuff.
9/10