In the chaotic Moscow of 1991, U.S. Ambassador Stephen Metcalfe is told that he's the only man who can keep the Soviet Union from falling into the hands of madmen. To find out why, we travel back to 1940 and meet a young Metcalfe as a James Bondian agent in Nazi-threatened Europe. The new book from the late Ludlum -- no longer an author, only a trademark -- feels like an espionage soap opera, complete with tortured romance, contrived plot twists, and overheated dialogue like ''Run, my darling. Run.'' (Who calls anyone my darling just after being shot at?) ''Tristan'' would still be a decent airplane read but for the fact that since the book is told in flashback, we know our hero will survive each hair-raising situation that Ludlum Inc. concocts.