The documentary maker Laura Poitras profiles William Binney, a veteran of the National Security Agency who helped design a top-secret program to collect Americans’ personal data. Binney explains how the program he created turned inward on this country. After resigning and speaking out publicly, he is among a group of N.S.A. whistle-blowers, risking his freedom, livelihoods and personal relationships to warn Americans about the dangers of N.S.A. domestic spying.