Do you know what the average life expectancy was in the 18th century? It was 17. (You read that right.) No, this wasn’t just about the fact that human beings back then tended to live less long. It was about the staggering inequality that society was built on. In Europe, the majority of people were hand-to-mouth laborers who drifted from place to place, lacking the benefits of being landed servants. (Not that being a landed servant was any picnic.) They existed in poverty, without health care or schooling or much of anything else. Their lives, in effect, were a death sentence.

That stark reality is the taking-off point for “Capital in the Twenty-First Century,” a nimble and eye-opening documentary that puts you in the revelatory position of looking back over the last 300 years — where we’ve been and where we’re going — from a God’s-eye economic view. That may sound dry as dust,...