Dr. Bragan is working on some important research for NASA when he is stricken down due to stress. To recover, he takes a trip to Japan to work on his insane theory: that humans evolved not from monkeys but from plants. He will prove this by turning a plant into a man, because science tells us that if one thing can be turned into another thing then we must conclude that the second thing necessarily used to be the first thing! The math is solid and so our movie begins.
Helping out Dr. Bragan is his lovely assistant Noriko and her hunchback, and some dogs. Dr. Bragan is basically the worst person ever. In every scene he is either hitting on Noriko in that creepy old-guy-who-thinks-he's-hip way, or he is totally flying off the handle for absolutely no reason at all. The movie even opens with him completely tearing into these guys and I never understood why he was so angry about everything.
This is pretty much your typical 1950s monster movie, complete with mad science and a final confrontation with a guy in a rubber monster suit at the end. And much like 1950 monster movies, you have about one hour of boring talking with everything that happens in the movie occurring in the last thirty minutes. Really the only thing differentiating this from, say, The Wasp Woman, is that The Revenge of Doctor X has a couple scenes of exploitative nudity.
Also, this movie takes place in Japan and surprisingly it has actual Japanese people in it speaking actual Japanese, and not a bunch of white folks squinting their eyes going "ahhhh sooooo domo arigato."
Other than the (comparatively) exotic setting, however, there is not much to recommend about this movie. The acting is bad all around and the movie is pretty lifeless all around. Also of concern is the fact that the title is a filthy lie. I don't think you could come up with a more inaccurate title for this movie, a movie which has neither a character named Doctor X nor any kind of revenge whatsoever in it. So this movie is lying to you right out of the gate, before you have even started watching it. That's like showing up for a first date and the person has not only lied to you about their name and also she isn't a human but is actually a robot dog.
The monster is cool, but you have to wait until nearly the end for the inevitable rampage and even then it's too brief and you end up feeling like you wasted an evening on a movie that would have been pretty unremarkable back in the late 50s, never mind 1970.