Season 3 (episodes #1-13)
★✯✯✯✯ - pretty bad
★★✯✯✯ - not so good
★★★✯✯ - pretty average
★★★★✯ - good
★★★★★ - pretty great
Susan Strong gets Finn and Jake to help her reclaim an underground city from evil monsters who have scared off the residents. I think Susan is a pretty boring and forgettable character, so episodes surrounding her are generally just passable to me. This is no exception.
'No One Can Hear You'
One of the creepiest episodes of the entire show, this one still gives me the chills when I watch it. After Finn gets knocked out by a deer, he wakes up some time later to find the entire Candy Kingdom is empty, with only Jake there wandering around the streets waiting for a surprise birthday party. The amount of tension this episode builds in only 11 minutes is astounding. I wouldn't want to spoil it for anyone, but the ending is great and every bit as unsettling as the episode leading up to it. One of the best episodes of the show.
'Jake Vs. Me-Mow'
Finn and Jake get brought in by Wildberry Princess to prevent her impending assassination. As it turns out, the assassin is a very small cat who hides inside Jake and threatens to kill him if he doesn't carry out the assassination for her. This episode could have easily not worked at all, and honestly the first time I watched it I wasn't a huge fan. But it's grown on me, and the ending is hilarious, unexpected, and totally genius.
A snow golem helps a firewolf pup return home at risk of his own safety. I'm going to save my thoughts on this for when I do my best of season 3 list, but let me just say this: it is really, really great.
'The New Frontier'
Jake has a dream that makes him think he's going to die. Finn tries to prevent this from happening. I know a lot of people love this episode, but I honestly don't have much to say about it. Never been a fan and I always forget about it. Might even make my bottom 5 list, who knows. Moving on.
'Holly Jolly Secrets, part 1'
The Ice King throws away a bunch of secret tapes, Finn and Jake find them and decide to watch them to see if they can figure out if he has any evil plans. The first of two, and does easily the weaker part of the story. Pretty much anything you have to say about the episode as a whole comes from the second one. But it sets up part 2 really well, so I can't complain.
'Holly Jolly Secrets, part 2'
Ice King finds out that Finn and Jake are watching his tapes and tries to stop them. Certain facts about Ice King are revealed. While part 1 takes its time and doesn't have much to offer, this one introduces one of the most interesting aspects of the entire show in its exploration of Ice King's backstory. Between this and 'Thank You' (and even the meh 'Fionna And Cake'), Adventure Time took some huge steps in season 3 that helped shape the creative direction of the rest of the series.
Finn and Jake are invited over to Marceline's house for a jam session, but wind up in her closet (I'll let you find out how/why) as she goes about her routine. I feel like this episode should have been a lot better than it is, and in several ways seems like it could have functioned as a great introduction to Marceline as a character with only slight adjustments. It's a couple seasons late, reducing the impact and tension it very well should have had.
Finn and Jake go to a library so Jake can study the history of rainicorns. Finn gets bored and wanders around until he discovers little people made out of paper who are fighting a great war against little people made of mold. This is a great example of an episode that exists. It has a beginning, middle, and end. And that's all. Very disposable.
Finn gets hurt and is told the only treatment involves being kissed by horrifying clown nurses. He rejects this and decides to find his own way to cure his broken bones. Finn episodes where he comes out ahead against all odds aren't usually my favorites ('Memories of Boom Boom Mountain', for example, was very bleh to me), but something about this episode is still really funny to me. It's nothing mindblowing, but still fun enough.
A weirdly underrated episode, this one sees Finn and Jake investigating the circumstances surrounding the death and identity of Ghost Princess. She can't remember who she was or how she died, which results in some very tragic revelations. I was touched by this story, even if it wasn't totally original and was pretty predictable in certain ways. The visual style of the episode was also pretty great.
Finn and Jake go through a dungeon specifically designed by their now-deceased dad to help toughen up Finn. It pushes Finn, but only in an emotional way, as their dad instigates Jake to insult him as it goes along. Definitely a solid dungeon crawl episode, with some cool monsters and a strong emotional core. I don't generally think of this as one of the best episodes in the show, but it's a really good one either way.
Finn is an emotional wreck over his unreciprocated love for Princess Bubblegum, so Jake goes on a quest to find him someone else to crush on, in hopes that he might finally get over her. This leads him to Flame Princess. The teen romance episodes of this show aren't always the very best, but when they're handled this well, I can't help but to admire them. This is one of the best episodes to illustrate how much Jake actually cares about Finn (which sometimes is genuinely called into question in others), and the introduction of Flame Princess and Flame King (Keith David!) was great. Very solid season finale, which is directly followed up on by the season 4 premiere.