Well, the story takes place in - *rataplan* - No. 6; an ideal and perfect City, without starvation, without wars and everything you need is right there. The only thing: the city is surrounded by walls.
Soon after the start we're introduced to one of the mains; Shion. A curious, naive young Boy who lives in no. 6. One night he lets a strange, injured boy into his house - and this boy is revealed as the second main named Nezumi (Side note: Nezumi is the jap. word for Rat).
Of course is Shion interested in Nezumi - who would not if hardly nothing happens in your life and everything seem to be predictable - and that is point where the story starts.
The setting was very nice indeed and gives the show a cool atmosphere. But yet, it takes almost the entire show to get the answer for questions such as "why it was built like this?", "why does Nezumi try to destroy it?", "why..." "what..." "how..." and so on.
Some parts of the story were extremly predictable, whereas others weren't at all; the Kiss for instance.
The animation was alright, but not outstanding. The same for the characters. They were memorable, but not special at all. They were explained, but at times I felt I was missing out something. Perhaps the novels have or give a better explanation. I assume that they would since 9 volumes were published. However, I did like Nezumis style most, followed by Inukashi "Dogkeeper. I think they gave a nice impression of how life could actually look like if you were an outsider.
Another thing what I extremely enjoyed was the attention for details in the background - the leafs moving in the wind, the light behind tiny windows far away - which gave the anime such a great atmosphere.
But, without a doubt, the animes strongest point was the Art/Sound. The sound of rain or wind for instance are very well done and the voices fitted the characters nicely.
The opening and especially the ending theme can be easiely described with one word: awesome! I became a huge fan of the ending theme because of the very nice sound and the beautiful lyrics. But I also liked the sound during the show - there were times when it was bizarre, there were times when it was quite soft - and I found myself paying more attention to the sound than the show. Having said that, I personally think I only keep the anime in my mind because of the sound.
So how shall I rate this anime? After considering all these points I would give the anime 7/10 rather than 6/10, just because of the sound.
