Herman is hit by an arcing electrical current while trespassing in Grandpa's lab one evening. Grandpa has him covered with a sheet when the family responds and reveals that the accident disfigured Herman. When the sheet is removed, Herman appears in the visage of actor Fred Gwynne, without makeup (and noticeably less bulky, too).
Herman must deal with his wife's and son's rejections at his appearance. When Lily suggests plastic surgery to restore his beauty, they take a trip to Dr. Dudley. Dudley, upon seeing the new Herman and a picture of what is requested states, "No plastic surgeon would touch him."
Grandpa finds the original blueprints from Dr. Frankenstein concerning Herman's "constrcution" and rigs his lab to place Herman into another electrical field that should restore him. The measure is only partially successful; Herman regains his pre-accident "appearance" but exits the procedure as a female!
Having taken a job as a cocktail waitress, Herman steps out into a rainstorm on his way to "work" and is hit by a chance bolt of lightning and thrown to the ground. He immediately recovers and realizes he is "male" once again --- the old Herman everyone loved. His family runs outside to share in his (and their) joy.
[How the bolt of lightning also changed his woman's cocktail dress into his standard mens suit and removed his lipstick, etc. is left unanswered.]