The story of Zetman starts off with talking about how there is an underground fight club found by the Amagi Corporation, where rich people pay to watch mutated monster fight to their death. It start out fine until one of the monster which is a wolf started to rebel and jumps to the viewing stand and kill everyone that is inside. That's where are main character Jin Kanzaki comes in, he was born when the place is falling apart he is the first mutated monster that's a human and can turn into a superhuman call Zet. His destiny is to eliminate all the mutated monsters that escaped the place.
After Jin grew up to an kid the first major event that he faced is his "grandfather", but is actually the scientist that saved him, then he got killed by one of the mutated monster. Then he meets Akemi Kawakami who lost her son and her husband, she took Jin in and treated him like it was her own son. On one night where Jin and Akemi were walking home from the supermarket Akemi got her face slash by one of the mutated monster that goes after Jin. Then Jin turn in to a incomplete form of Zet and killed the mutated monster. After Jin grew up to an adult he kept his duty to eliminate all the mutated monster, on one of the day he was on duty he meets Kouga Amagi his old friend who he goes around with when he was little to help other people.