With the help of doctors and scientists in America, Germany and Britain, Horizon unpicks the mystery of the Moscow theatre siege. In October 2002, Chechen terrorists took a thousand people hostage in a Moscow theatre and threatened to kill them. The problem was how to get them out alive. A bloodbath seemed inevitable. Three days later Russian special forces stormed the theatre using a secret gas to knock everybody out. 129 hostages died - apparently killed by the very gas that was meant to save them. Horizon investigates the mystery substance, and why so many died. The Russian authorities insisted their secret weapon was not lethal. The claim provoked contempt from the victims families, and incredulity among doctors and scientists around the world. But were the Russians actually right? The Russians offered just one clue. And in Germany there was a scientist who had the means to test it: a urine sample taken from one of the survivors shortly after he was freed. Horizon follows as extremely sensitive tests are performed to find out if the Russians were telling the truth, and uncovers a deeper secret.