Goodbye Gemini (also known as Twinsanity) is a 1970 British psychological horror film directed by Alan Gibson and starring Judy Geeson, Michael Redgrave, and Martin Potter.

[3] Based on the novel Ask Agamemnon by Jenni Hall,[4] it concerns a pair of unusually close fraternal twins, Jacki and Julian, discovering Swinging London while home on Spring Break.

It was released concurrently with Freddie Francis' Mumsy, Nanny, Sonny and Girly (1970), another horror film which also dealt with an unusual familial relationship and contained a scene implying consensual brother-sister incest.

[citation needed] Jacki and Julian Dewar, a pair of fraternal twins, arrive via bus in London on Spring break from university, while their father is in Mexico on business.

The pair launch themselves into London's underground party scene, clubbing at strip bars, accompanied by Jacki's teddy bear, Agamemnon, whom the twins address – and regard – as a father figure.

Meanwhile, Denise reveals the plan to Jacki, telling her that Clive has similarly raped and blackmailed other men in the past, going to far as to sell some of them into sexual slavery when they were unable to pay him.

Her discovery of Clive's body upon returning home inadvertently causes a citywide manhunt for both siblings; James lies to the police about Jacki's whereabouts the night of the murder, allowing blame to be shifted solely onto Julian.

Deducing that Julian has gone to hide in the same hotel where Clive kept his Circus, Jacki tells James that she'll convince him to turn himself in and to call the police if she hasn't returned after an hour.

The film makes significant divergences from the book, which was highly experimental in nature and involved the use of dream and fantasy sequences written in the style of a Greek tragedy, during which Agamemnon comes to life and interacts with Jacki.

Most notably, the film presents the story in chronological order, whereas the book takes place within the frame narrative of an amnesiac Jacki slowly piecing together the events leading up to Clive's death as she convalesces at James' house.

And Judy Geeson and Martin Potter are too normal to make the central semi-incestuous relationship convincing, despite the arty shots of them reflected in mirrors and their over-furnished house stuffed with exotic bric-a-brac.

Despite the best efforts of Geeson and Freddie Jones as a camp antiques dealer, wooden performances and an inconsistent script make the film little more than an interesting, if rather dubious, 1960s snapshot.