When Ruben Cavazos changed his clothes at daybreak, he was no longer a CAT scan technician at the University of Southern California Medical Center. He became the man known--and, in a few special cases, feared--as Doc, international president of the Mongols, the fastest-growing and most closely watched organization of its kind in the United States.

In reality, the Mongols are a tightly knit band of brothers devoted in equal measure to their club, their fellow Mongols, and their freedom. They live to enjoy life, party, and travel the open road. Above all, they demand respect. When...