The Enterprise is sent to answer a distress call from Ephrata IV, one of the Federation's most important research posts. Arriving and receiving no response to hails, Kirk, Spock, Sulu and security officer Ensign Fawzia Yaseen beam down only to find the post staff have been brainwashed into serving a fanatical religion, the Truth. The outpost has been taken over by the Crusaders, the militant religious order of the Ialatls who have come from another dimension.