Calvin Zabo was born in Trenton, New Jersey. He was a morally abject but brilliant biochemist who was fascinated by the effect of hormones on human physiology. One of his favorite stories was Stevenson's 1886 classic, Strange Case of Dr Jekyll and Mr Hyde. He was convinced that the experiment Dr. Jekyll performed in the story could actually be accomplished, and became obsessed with the idea of unleashing his full bestial nature in a superhuman form. However, he needed money to do this, so he robbed his various employers systematically. Though too intelligent to be caught, the medical community was suspicious of his tendency to always be employed by organizations which were subsequently robbed. Zabo eventually sought work as a surgeon in a hospital where Donald Blake was a directing physician, yet Blake would not allow him a job due to his history. Zabo was enraged that Blake would not give him the position, even though he did indeed intend to rob the organization, and swore revenge. He was eventually successful in creating his formula and turned into a huge, Hulk-like creature he called "Mister Hyde," named after the character in the novel. In this new form, he found he had superhuman strength enabling him to crush cars and tear through steel as though it were made of cardboard. With his new superhuman powers he sought out Blake, whom he tried to kill by throwing him from a window, but Blake transformed into Thor by striking his cane on the wall and survived, claiming he had saved Blake. Hyde, hearing of this on the radio, decided to eliminate Thor, and tried to frame him for a bank robbery by using his superhuman strength to tear open a bank vault while disguised as Thor. While Donald and Jane were out, Mister Hyde met and kidnapped them at gunpoint. As Don Blake, Thor was tied up next to a bomb that would explode in 24 hours unless Mr. Hyde defused it. However, when attempting to steal a Polaris submarine to roam the seas like a pirate, Hyde was defeated and the authorities, seeing his superhuman strength, realized he must have impersonated Thor, but Thor was forced to allow him to escape, as Jane Foster thought Blake was still in danger.