At the end of his fascinating new documentary The Unknown Known, director Errol Morris asks his subject a question the audience has long been wondering: "Why are you talking to me?" And his subject, former Secretary of Defense Donald Rumsfeld, admits that he's not really sure. Ol' Rummy seems to want to get something off his chest about his involvement with the Iraq War, but actually saying flat out that very bad mistakes were made is anathema to him, and he frequently tries to steer Morris' questions in other directions. (Though never quite as dickishly as in clips from old press conferences, in which Rumsfeld berates reporters who ask reasonable questions.) As is typical for Morris' films, the talking head is augmented with archival footage, new graphics, and gorgeously crisp cinematography, with Danny Elfman providing a score that's a few arpeggios away from being a Philip Glass tribute. Much of The Unknown Known's structure comes from the many thousands of memos Rumsfeld wrote over the years — "snowflakes," he calls them, prompting almost too-perfect Citizen Kane snowglobe imagery — and there's some excellent file-retrieval porn for the Archives and Records Administration nerds. But beware Rumsfeld's smile: It seems natural enough, but it's simultaneously very disturbing, like he's laughing at things we can never know.