Though not one to interfere (much), the Watcher told the Fantastic Four of the menace Molecule Man posed and sent them back to New York City where the villain had already negated all natural laws of physics! The team jumped into action, but did not stand a ghost of a chance against the supreme power they faced.
Retreating to formulate an actual plan, the FF left Molecule Man to his own devices. Enraged, Reece held the entire city of Manhattan captive and ordered them to find Mister Fantastic, the Human Torch, the Thing, and the Invisible Woman.
Thanks to some unexpected help from a Yancy Streeter, the team made their way to Alicia Masters' apartment where Reed Richards realized that Owen's powers had no impact on living organisms. He hatched a plan that involved Alicia covering the FF in a thin layer of plaster before calling Molecule Man and tricking him into using his powers on it. Somehow Reed correctly theorized that doing so would cause a feedback that incapacitated the villain and caused him to drop his wand!
Using his stretching abilities, Mister Fantastic quickly snatched up the weapon just as the Watcher opened a portal and dragged Molecule Man away. Uatu thanked the heroes, returned the Big Apple to its original state and disappeared.
As we later learned in 1974's MARVEL TWO-IN-ONE #1 by Steve Gerber and Gil Kane, the Watcher placed Molecule Man in an alternate reality where time moved faster. He created a being who thought it was his son. When Reece seemingly died, the "son" spent his days building machines to help exact his revenge. Eventually he recreated the experiments that granted Owen his powers complete with the wand and ran into Thing and Man-Thing, but ultimately shriveled up because of the time difference between realities.