Mister Zero was a criminal scientist who in the process of experimentation could no longer breath air at room temperature or hotter. Wearing a suit to cool him down, Zero devised a cold/heat gun which he used against the Dynamic Duo. When a steam pipe is broken during a battle with his foes, Zero's ability to live under normal conditions is cured.
Sometime later, Dick Grayson's Aunt Harriet needs the felon's weapon for cryosurgery. When Batman and Robin attempted to retrieve it, they ended up reviving their rivalry and in the process Zero...now as Mister Freeze...once more could no longer live a normal life. At this point, he became an irregular and highly dangerous enemy.
This all transpired on Earth-One...on Earth-Two Robin did not have an aunt so Mister Zero stayed retired and never reverted to his heat-affliction.