It's a really simple, short show so I'm going to do a really simple, short write up. The whole show (which is 13 episodes long) only takes about an hour to get through. Each episode runs roughly 5 minutes. So if you've got an hour to kill, I recommend it, because it's very cute and fun. Watching them individually, the story feels a little choppy as there isn't enough time in a single episode to really cover a lot of ground. Watching them all together should remedy this. It's not the most fantastic show, but it's a decent watch.
In Chocolate Underground, a political faction called the Good For You party has taken over. I'm not sure how they managed to get voted into power to begin with, but it's kind of irrelevant. The Good For You party has outlawed chocolate, and basically anything that isn't strictly health food. They've replaced it with government sanctioned recipes and food items. To hunt down law breakers, they have built giant robotic sensor machines called CACAO. They can detect chocolate (but not its raw ingredients), and they also serve as weapons when needed. The two main characters, Huntley and Smudger, love chocolate and want to bring it back to their home town. They believe that chocolate brings happiness and kindness into the world. With the help of their friends Louise and Blades they form an underground chocolate ring (sort of like underground bars during prohibition in the '20s in the US) and attempt to uncover the secrets of the Good For You party and expose the corrupt government to the world.
Told you it'd be a quick look. I've been working hard on my Fable 2 Pub Games to get all the goodies I can before the game hits next week. But I've also been watching a lot of stuff on the Funimation channels, which I would love to tell you all about once I've seen some more episodes. Also I'm just about through with the summer season, and I'm starting to get episodes together to decide what I'll be watching this fall. A few are a sure thing (Clannad After Story, Skip Beat, Vampire Knight Guilty, Nodame Paris Chapter), but the rest is all up in the air right now until I see some episodes.
Oh, and just for reference, CU is from Production I.G. It's based on a manga by Aiji Yamakawa, which was itself based on a children's novel by Alex Shearer.