Rangers and Celtic share a stadium city and a hatred that outruns any ordinary sports rivalry, playing out in violence, sectarian chanting, and slurs that stretch back to Glasgow's Irish Catholic and Scottish Protestant divide. The film treats the Old Firm fixture as a case study in what football can do to tribal loyalty, tracing how a match between two clubs became tangled up with religion, national identity, and generations of grievance. Fanzine editors from both sides sit down to explain the codes and rituals of support, the songs that mark out allegiance before a ball is kicked, and the lines that get crossed on derby day. The crew travels with Celtic's away support, the Bhoys, to a match at Tannadice, riding along with fans whose loyalty reads less like enthusiasm than inherited obligation. Interviews and match footage build toward a plain question underneath all the noise: how a game keeps producing this much loathing, season after season, without anyone quite agreeing on who started it.