Martin Kemp created the Human Flea in an attempt to save his grandad's ailing flea circus, Smallest Show On Earth Flea Circus East 89th St. His petty crime wave caught the attention of Batman, whom he escaped from in their first encounter.
The sadistic killer Kadaver raided the flea circus to unleash a swarm to kill Gotham and killed Grandad Kemp. Martin found the body and saw Batman nearby and assumed that Batman had been the one responsible. Donning his Human Flea costume he attacked Batman knocking him off a building and kicking him in the spine before Batman used a batarang to disable the Human Flea.
Martin explained his actions to Batman and reluctantly teamed up with him to catch Kadaver who was going to unleash the bacillus plague but was thwarted.
Human Flea appears in a dream sequence helping Batman defeat a horde of villains after Batman is gassed by Narcosis