From the Chaosium website:
Mystic Alliances offers rules for using teenage investigators in a Call of Cthulhu setting in the near-future. During the years 2009 to 2011 the mythos again stirs, most notably, in Seattle. This has resulted in teenagers taking the front roles in the fight against and discovery of the supernatural — while older characters operate in the background as Mancers, characters who oversee the teenage investigators. The time period when the game takes place is a period known as The Darkening, the approximately fourty months preceeding the Dawning, a time that comes during the year 2011.
After the Dawning humankind is besieged by a number of mythological threats which you, as a teenage investigator, must confront in order to keep society safe and sane.