Six months after the events in Trails in the Sky Second Chapter, Kevin Graham finds himself in the airspace of the Erebonian Empire, right in the middle of a mission for the Septian Church. After a successful completion, he returns to his ship and immediately receives another request, now to investigate an artifact recovered from the bottom of Valeria Lake in the Liberl Kingdom. There, he meets Ries Argent—an old friend—and they both get caught up in an incident that leads them into Phantasma, the Land of Shadows, a place that seems to follow its own otherworldly rules. Thus, they begin their investigation, which leads them to unearth their own past.
Source: Wikipedia, "The Legend of Heroes: Trails in the Sky the 3rd", available under the CC-BY-SA License.