The Thing is wandering around in the rain feeling sorry for himself because he thinks he's been rejected by his love. He stumbles across a man that seems to be sympathetic to Ben Grimm's concerns and problems and seems to give him a sympathetic shoulder. After a drugged cup of coffee, this scientist uses his "Duplication Apparatus" and makes himself the Thing while returning Ben Grimm to his human form. This Scientist plans to take revenge on Reed Richards, who he believes to be a glory hound. After convincing the Fantastic Four that he's the real thing and the person claiming to be Ben Grimm is in fact a fake, he plots his revenge on Reed Richards.
Note: This issue reprints one of the most popular Fantastic Four issues, if not the most popular of all! When one looks at top 10 lists for Fantastic Four issues, #51 appears quite often.