Errol Morris’s background as a private investigator might explain his unrivaled ability to gain access and answers, to know when to push or pull back, how to dig and where, what to look for, how to supply interviewees with just enough rope to let them hang themselves. Alex Gibney has this ability, to a lesser degree, as does Ken Burns. But they take their cues from Morris, who, in turn, took many cues from Werner Herzog.

Morris’s most recent documentary, The Unknown Known, focuses almost entirely on the labyrinthine Defense record of Donald Rumsfeld, in much the same way Morris’s Fog of War focused on the record of Robert McNamara. While not an overt indictment of Rumsfeld (one could actually make the case this film is a tribute to Rumsfeld’s genius) The Unknown Known does an exceptional job of exposing Rumsfeld’s blind spots. As an interviewee, Rumsfeld shucks and jives, conversationally perjuring himself with all the deftness of a swashbuckler.

The name of Morris’s film is derived from a 2002 press conference. Rumsfeld, while responding to a question about terrorist ties in Iraq, insisted: “There are known knowns. There are things that we know that we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns, the ones we don’t know we don’t know.”

Here we find both the genius and the conundrum – an equivocator so complex he steered a country into war. Morris conducted over 30 hours worth of interviews with Rumsfeld in conjunction with the film, and Rumsfeld – to his credit – breezed his way through every minute. He’s a marvel to behold, like watching an aging champion lace ’em up for one last spar. The Unknown Known is a compelling character study, so rich that while one may or may not come out of with a more sympathetic view, there’s very little risk of coming out of it with no new view at all.

(The Unknown Known opens in limited release this Friday.)