Jason Bourne returns to Georgetown University and the mild world of his alter ego, David Webb, hoping for normalcy. But after so many adrenaline-soaked years of risking his life, Bourne finds himself chafing under the quiet life of a linguistics professor.   Aware of his frustrations, his academic mentor, Professor Specter, asks for help investigating the murder of a former student by a previously unknown Muslim extremist sect. The young man died carrying information about the group's terrorist activities, including an immediate plan to attack the United States.   The organization, the Black Legion, and its lethal plot have also popped up on the radar of Central Intelligence, where new director Veronica Hart is struggling to assert her authority. Sensing an opportunity to take control of CI by showing Hart's incompetence, National Security Agency operatives plan to accomplish what CI never could-hunt down and kill Bourne.   In Europe, Bourne's investigation into the Black Legion turns into one of the deadliest and most tangled operations of his double life-the pursuit of the leader of a murderous terrorist group with roots in the darkest days of World War II-all while an assassin as brilliant and damaged as himself is getting closer by the minute . . .