Office 119 is an international Intelligence agency.
In the wake of the September 11 attack, high ranking officials in the U.N. had serious doubts of the American ability to deal with such evil. They had no doubt that the evil was stronger than ever, though, so something needed to be done. If the most powerful country in the world couldn't stop it, perhaps a select group of highly trained individuals, acting outside of all political boundaries and with almost unlimited resources might.
Thus was born the highly secret agency called Office 119. Its charter was to fight for justice anywhere in the world regardless of the whim of whoever was in power at the time. To keep their agents freer from influence, they chose only dead people. Naturally, they weren't dead in reality but were people that were thought dead by all who knew them and who were officially deceased on paper. All ties to the previous life was severed and the agents were able to operate a lot easier because they didn't officially exist.
One of the key agents for Office 119, and the true protagonist in the series is a woman who goes by the name Renate Bachle. Formerly a German member of their equivalent of the FBI, she chased a powerful group of bad guys until they killed her. Now she can really go after them.
Though Bachle is very important in the series and remains the most interesting of the agents, the series is still mainly about the agency as a whole and the work they do to fight terrorism without being noticed.