In the 1990s, Paul Romer revolutionized economics. In the aughts, he became rich as a software entrepreneur. Now he’s trying to help the poorest countries grow rich—by convincing them to establish foreign-run “charter cities” within their borders. Romer’s idea is unconventional, even neo-colonial—the best analogy is Britain’s historic lease of Hong Kong. And against all odds, he just might make it happen.