One can easily tell the plot from the title. A head living without a body, or on another body.
In this movie, it is both.
This is a "mad scientist" film.
The reason this works is because science fiction is "science fiction", so we don't worry about the "unrealistic" premise of a head being attached to a different person's body.
That's because it is just "one premise." Only one item to suspend belief over.
And good science fiction, in fact, any good Fiction, is "credible characters in incredible circumstances".
Here, the "circumstance" is really the one "mad scientist". The "head" is just a "symptom" of the "disease".
The difference between the "poor" and "superior" mad scientist movies is the other characters. The poor movie will have the mad scientist simply being a god that no one but a hero and heroine can stand up to.
This film is a "superior" mad scientist film, because there are many characters who react to the lunatic in their own way. The film is a great blend of the suspense and horror along with the characters who eventually come to realize the man with them is insane.
