I'm beginning to develop a real aversion towards alleged cult-films that feature the words "...in swinging London..." in their synopsis. It seems like "swinging" is simply a poor excuse to bring forward insufferable lead characters, nonsensical and totally unrealistic storylines, and ridiculously random plot twists. "Goodbye Gemini" is one of the most disappointing movies I've ever seen, especially because it was coming from an adequate director (Alan Gibson of "The Satanic Rites of Dracula") and starring one of the most gorgeous actresses of British horror cinema (Judy Geeson of "10, Rillington Place" and "Fear in the Night").
Based on a book that I luckily haven't read - it's probably even ten times more boring than the film - the story revolves around twin siblings, Jacki and Julian, arriving in London. They are naïve, childish, and unnaturally obsessed with each other. Just because they are so 'otherworldly', annoying hippie Clive hopes to take advantage of them, but then the twins turn psychotic.
The first full hour of "Goodbye Gemini" is dull, irritating, and pointless. Then comes the one and only horror-highlight. And then the last half hour is terribly boring again, with a miserable climax.