The hatred between Glasgow's "Old Firm" sides is the most famous in world soccer. It's the game's flagship loathing, proof of the power of the sport to inspire profound levels of tribal loyalty and a near-Pavlovian revulsion at anything to do with a rival. In the first episode of our documentary on Rangers and Celtic, we try to get a handle on the political, religious, and national identity clashes that have shaped the rivalry, speak to fanzine editors on both sides of the divide and travel with the Bhoys' away support to a match at Tannadice.