One day while a construction worker was hauling some lumber up a skyscraper, a earthquake hit unexpectedly. as the sky scraper had not been finished building it fell over, with the construction worker at the top. As expected the worker died, but woke up again in a white space. Apparently somebody in another world burnt up magic trying to summon a demon (and failing), so she was going to send him to that summoner, let him keep his free will, and to further humiliate the summoner send him in the body of a slime. So that he would accept she sent him in a slime form that can keep evolving no matter what, warning him that the body was still experimental.