Showtime’s “In the 20th Century” is a millennium-related strand of feature-length documentaries in which famous directors take on major subjects of their choosing. In the last of the six films, “The American Tapestry,” filmmaker Gregory Nava takes viewers on an uplifting and challenging journey through the memoirs of five immigrant families, each one on a quest for its own American Dream. Beautifully interweaving accounts from several generations, Nava composes an astonishing tapestry of personal triumphs and tragedies, as each story of courage unfolds. The American Dream is an elusive thing, and the lives of the people in Nava’s film are both triumphant and tragic, teeming with optimism and sometimes despair. They expose the finest and worst in America as well as what we feel most magnificent and dreadful. They are part of the many contrasting threads that make up the American tapestry — a complex portrait of a nation at the turn of the millennium.