Well, first off, it's actually Frankenstein's granddaughter. But who cares about such details? Though she keeps mentioning her grandfather's experiments, technically it is true that while she's Frankenstein's granddaughter, she is also Frankenstein's daughter. But her father didn't continue with her grandfather's work. So...
Anyway, for the first five or ten minutes, this movie is a horror film. It introduces Frankenstein's granddaughter and her brother, and tells us that they've killed several of the neighborhood children in the name of science. Then for the next half hour it's a western. We meet Jesse James, and there is an attempted stagecoach robbery. It's actually not a bad western.
Then more than halfway into the film, Jesse James meets Frankenstein's granddaughter, as he seeks medical attention for his mate who's been shot. Frankenstein's granddaughter falls for Jesse James, but he's not interested. So like any psychotic woman, she decides to have him arrested and hung.
The movie is pretty silly. But come on, what would one expect from something with such a title? It's certainly one of the oddest Frankenstein films, and for that reason, it might be worth watching. You know, if there's nothing else on. And you have a significant amount of alcohol in your bloodstream.