In 2009 Jim McKelvey lost a sale because he couldn't accept American Express cards. Frustrated by the high costs and difficulty of accepting card payments, McKelvey joined his friend Jack Dorsey (the co-founder of Twitter) to launch Square, a startup that would enable small merchants to accept card payments on their mobiles. But just as Square was taking off, Amazon launched a similar product - but retreated within a year. How did Square beat the most dangerous company on the planet? Was it just luck? McKelvey eventually found the key: a strategy he calls the Innovation Stack.