The testimony of people who have fled territory controlled by Islamic State (IS) reveals how the extremist group, in the name of religion, uses terror to maintain its power. Via the activities of an anti-IS group formed by young Syrians forced from their homeland, and from internal IS videos, we learn how IS used elaborate stratagems to placate local tribes and obtain control over the oil fields that fund its activities. Meanwhile, IS slaughters women and children in tribes that refuse to cooperate. The anti-IS group seeks out the testimony of survivors, to indict IS on the international stage. What drives the IS fighters to such fanatic behavior? Interviews with former fighters reveal that IS follows an extreme interpretation of Islamic law and indoctrinates its fighters to treat all opponents of IS as "apostates" who should be killed. We also hear a behind-the-scenes report from a man who worked in the IS media operation, revealing the strategy behind the group's propaganda videos. Through interviews with Syrians and Iraqis who struggle to regain their homelands, and with people who have been involved with IS, the reality of conditions in IS-controlled territory comes into focus.