The documentary "The Unknown Known" is an extended cat-and-mouse game, but one in which each side thinks he is the cat.

Oscar-winning director Errol Morris, famous for pinning down Robert McNamara in the documentary "The Fog of War," turns his attention to former Secretary of Defense Donald Rumsfeld in "The Unknown Known."

Rumsfeld was famous for saying that "there are known knowns; there are things we know 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." Morris supplements that with a 2004 Rumsfeld memo where he adds the idea of unknown knowns: "things that you think you know that it turns out you did not."

That’s a notion that comes into play repeatedly in "The Unknown Known," which arrives for home viewing today; much of the film has Rumsfeld facing the camera, answering Morris’s offscreen questions, often in swaggering and bantering fashion. While Rumsfeld’s study of history has led him to see numerous failures of imagination -- going back to at least Pearl Harbor -- it is clear that Rumsfeld’s imagination is vast and powerful, especially when it comes to justifying his own actions.

With a smile on his lips that for the most part does not reach his nearly closed eyes, Rumsfeld bobs and weaves, trying to turn questions back on Morris, and in at least one case offering a flat denial that Morris promptly undercuts with an archival clip of Rumsfeld saying precisely what he has just denied. Unfortunately, Morris does not call Rumsfeld on that in person, although there are other times when Morris’s incredulity is evident.

Instead, we try to get past what is known about Rumsfeld to what is indeed unknown, including what he chose to know. And that’s where the film is most interesting: How did one of the most important people in the country think through the major issues? Helping Morris in that process is Rumsfeld’s copious memo-making, which was so frequent that his notes became known as snowflakes.

But even when faced with his own words, Rumsfeld labors to keep parts of his thinking unknown. And the film never quite gets past some of his walls. Even if it did, Rumsfeld would likely offer one of those unnerving smiles -- and another wall.

Extras include audio commentary by Morris, a separate conversation with the filmmaker and other elements.