"The Mysterious Molecule Man!" (X-419) by Lee and Kirby; 22 pages.
A freak accident gives a disgruntled atomic lab technician the power to control the very molecules of matter. He attempts to use his power to rule the world, but the Fantastic Four discover his weakness and stop him. The Watcher removes him for safekeeping.
First appearance of the Molecule Man.
Reprinted in Marvel Collectors' Item Classics #14.