During the 1940s, George Zucco made a ton of horror films--most of them for crappy little studios and with microscopic budgets. However, occasionally he'd appear in a decent film--one with higher production values and plots which made a bit more sense. This is the case with "The Mad Ghoul", as Zucco appeared in a film by Universal--a studio that made horror films just a bit better than everyone else.
Ted is in love with Isabel. He apparently can look past her annoying singing (it's very operatic and you either like it or hate it--most folks today would hate it). However, Ted doesn't know that the professor he's working for, Dr. Morris (Zucco), is a maniac who will do anything to possess Isabel. Eventually, Morris uses a gas he's created to turn Ted into a maniac who will do whatever the doctor tells him---including kill. What's next? See the film.
The acting is a tiny bit better than the usual B and the plot, though a bit silly, quite enjoyable if you like this sort of thing. Worth seeing.