Jon Hall plays the invisible man again, but he's a different invisible man than he was the last time. Like The Invisible Woman, this doesn't appear to be directly related to the rest of the series, but it is closer, and it is the only film since the first one that fits into the horror category. The invisible man is named Griffin but is apparently unrelated to the family from the other films in the series. Also, the means of invisibility is a drug, but it is invented by a different scientist, and Griffin is simply the guinea pig... a guinea pig who decides to take advantage of his new invisible state. This time it isn't the drug that causes madness, Griffin is already crazy when the movie starts, and it's a good twenty minutes until he becomes invisible.
Robert Griffin has just returned to London. At some point he was in Africa. Whether that's where he's coming from is debatable, but wherever he's coming from he's just recently killed three people while escaping from a mental institution. He's in London to call on some old friends and collect his half of the profits from an expedition they collaborated on in Africa. Unfortunately, they thought he had died on that expedition, and have lost all the money, including his half. Unfortunately for them, he believes that the reason they were so sure that he was dead is that they tried to murder him. They pretend to be reasonable but seem to be just stallling, which of course doesn't work with Griffin since he refuses to be reasonable, so they throw him out and warn the police that a wanted fugitive is in the city. As luck would have it, Griffin stumbles on a local scientist who's invented an invisibility formula and has been anxious to try it on a human. Griffin volunteers, then skips out on his new friend to use his new power to torment his old friends. Things go his way until he needs to be restored to a visible state and discovers the only antidote, a temporary one at that, is human blood. A full body's worth. Of course a madman wouldn't let a little thing like that stop him, now would he? What kind of madman would he be if he did? All the other madmen would laugh at him and call him names. And do you think they'd let him join in any of their madman games? No, they wouldn't.
The Invisible Man series finally gets back to what made the first film great, and finishes in style. Jon Hall is better as a villain than as a pompous hero, and Evelyn Ankers is her usual self, but John Carradine is what makes this really worthwhile. If only all of Universal's monster series could have ended this strongly rather than being run into the ground.