Jin Kanzaki is a boy raised by his homeless grandfather. He isn't a common boy, though. He has a halo makr in his left hand and is very strong. He uses his strength to help people, charging 10000 yens for the job. What he doesn't know is that his grandpa worked as a scientist for a screct club where rich people bet in fights between humanoid monsters called players. But one day the players acquired conscience and turned against their former masters. This past comes back to haunt the characters and Jin will find himself part of a conspiracy that involves the players and the former organizers of the fights. He'll also have to deal with his true nature.
It's pretty hard to do a synopsis because the anime tries to fit 17 volumes of story into 13 eps and I didn't want to spoil the readers.
Technically, the anime is good. The animation is good, which is important in an action anime. They kept Katsura's original character design, which is very good, though sometimes they get sloppy with it and the characters look strange. The soundtrack is pretty average, but the OP and ED themes are terrible. The voice acting is ok, with nothing good or bad to mention.
I read the Zetman manga, so I already know most of the plot and I like it. But since it's only 13 eps and the manga is still running, they'll probably create an original ending. Let's hope it's good.
Unfortunately, the anime has a very serious problem. It's too rushed. I know they can't possibly fit everything so far into 13 episodes, but it's really decreasing the anime's quality. Zetman has it's fair share of drama, but you just don't have time to relate to it, because it keeps throwing stuff at you without giving you time to react. The same goes for the characters. Zetman has a lot of characters and the anime didn't cut anyone so far. This gives each of them little time to make an impression, so you end up without being able to care about what happens to them.
I'm pretty disappointed with the anime, but I hope it's successful. Maybe it'll generate enough interest in the manga and, in the end, we'll have a faithful adaptation later, like Hellsing had.