Goodbye Gemini (also released as Twinsanity) is a 1970 British psychological horror film directed by Alan Gibson and starring Judy Geeson, Michael Redgrave, and Martin Potter. Based on the novel Ask Agamemnon by Jenni Hall, it concerns a pair of unusually close fraternal twins, Jacki and Julian, discovering Swinging London while home on Spring Break. Their experiences complicate the pair's relationship, which is already strained due to Julian's incestuous fascination with his sister, which he sees as a natural manifestation of what he believes to be the pair's hive-minded nature.