The story begins with the introduction of the STN-J team, a group of witch hunters in Japan who work in a corporate-office type of environment and who go out to do fieldwork to track down dangerous witches. Each of the members has a unique talent: Robin uses pyrokinesis, Miho uses psychometry, Michael is a master hacker, etc. Robin is the newest member and was transferred from Italy. Because of this her Japanese is a bit strange, although unless you watch the dubbed version you miss some of the jokes regarding that fact. She is, however, an extremely powerful person once she has some training.
These 'witches' are people born with various powers, who are said to become power-obsessed and deranged from using them. In many cases, those who this STN-J team have hunted are murderers, thieves, etc. They do not kill them outright (although a few characters do die), but use special Orbo bullets before the witches are taken by another STN-J unit. They do not know what happens to them; only that the witch is imprisoned where they cannot hurt others.
At first, the series seems to be the Creature of the Week type of story, a new witch hunted in every episode. The pace is slow through the first fifteen episodes or so. But as you pay more attention to how the characters respond to one another and work together, you realise that this slow pace is the natural progression of how real people work together and learn to trust each other. And that trust is paramount; the last episodes reveal why Robin was sent to the STN-J, and why she is the replacement for the last person to leave... and also, the fate of future witches. The ethical debate of people working for the STN with powers vs. people not working for the STN with powers is brought up; there is no difference between "humans" and "witches" who have powers, only how the STN perceives them. The pace of the series at the end is much faster as the heart of the story picks up.
The ending I found unsatisfactory, but it seems fairly standard of Japanese movies. There is no 'wrap-up'; it just... ends, leaving you forever wondering what happened. There are only 26 episodes, so it isn't a long anime (about standard: xxxHolic and Cowboy Bebop were about the same) and it's perfectly watchable within a week or so.
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