At an unspecified point in the future, the world has gone through some drastic changes. Humans mostly live in walled cities with numeric designations and most of the world you and I know appears to be underwater. Military force has nominally been banned. Twelve-year-old Sion lives in Chronos, the upperclass sector of No. 6, and along with his friend Safu attends a special school for the elite. Like in most utopias, however, Sion is somewhat discontented with his life, imagining himself floating through the sky and standing outside to scream into storms. This puts him in a way to rescue Rat, a scruffy escapee from one of No. 6's holding facilities. Rat is also twelve, and despite his initial mistrust of Sion, the two form a close friendship over the course of one night. Unfortunately for Sion, the totally-not-the-military arm of No. 6's government finds out what he has done, and as punishment Sion and his mother are stripped of their rank. Now, four years later, they live in Lost Town, the lower class sector, and Sion works for the parks department while his mother runs a bakery. Sion still questions some of the workings of the city, however, a dangerous practice which results in his being forced from the city itself and into the West District, a shantytown in the ruins of an old city just outside the walls. Rat is instrumental in Sion's escape, and the two begin to live together as they struggle with their different ideas of what changes need to be made in the world.

On the surface, No. 6 isn't much different from other post-apocalyptic science fiction. It has the dystopia disguised as a utopia, the malcontents working from the shadows, and the shunned members of society living in disgrace. Where is succeeds, however, is in the small details. The West District is full of the dilapidated remains of a not-so-distant past. Flies cover food in the markets. In the city, the government's Big Brother style tracking is done through ID bracelets that also serve as phones and computers, something that should make us think of GPS-enabled smart phones. Later on, as the plot comes to its climax, the brutal methods of No. 6 bring us clear reminders of the Holocaust in their treatment of the denizens of the West District, children included. These images, particularly a few in the last couple of episodes, really bring home the ideas that the story is trying to convey, and if other parts of the show have less impact, the visual symbolism is at least well done.

Another symbolic area the show succeeds in is through the use of literature. Rat's underground apartment is a library of banned classics, a nice Ray Bradbury reference in itself, and several titles are shown frequently. The two most important are Shakespeare's play All's Well That Ends Well a small message of hope, Oscar Wilde's fairy tale “The Happy Prince.” This is a story of self-sacrifice, very similar to Shel Silverstein's The Giving Tree, and its themes ultimately prove symbolic of the actions of several characters. Like both of the aforementioned tales, No. 6 does have that same sweet sadness towards the end, and the tragedy of one of the characters brings home the messages both Silverstein and Wilde convey. This is not to say that familiarity with either work is required, but it will enhance your appreciation of at least one character.

In general the backgrounds are more visually fulfilling than the animation, although some moments showcase a wonderful fluidity. The sub is a little stronger than the English language track, although that shows significant improvement once Rat, Safu, and Sion reach sixteen in episode two. By the end of the series, Greg Ayres provides a very good Sion, and if Kalob Martinez doesn't get quite the range of emotion from Rat as Yoshimasa Hosoya, he still does a very listenable job. It is worth noting, however, that he does not sing Rat's songs – that is left in the original Japanese even on the dub track.

No. 6 is a solid entry into the genre of post-apocalyptic science fiction, even if the ending feels a bit unfinished. (This is likely due to the fact that it is based on a series of novels; presumably the story continues beyond what was animated.) The show has an interesting plot, good character interactions, an ominous air, and a light shounen-ai element to it, though this should not scare off those who dislike the genre. Sadly beautiful in places, hauntingly violent in others, No. 6's eleven episodes worth your time.