A couple who seem to despise each other, living above a dusty old antique shop his dying father owns, spice things up by pretending to be the killer Dr Crippen and his wife or lover. With their fantasies, there’s some sort of connection, but when the costumes come off, there is none. He is weak and aimless; she wields all the power and uses it to belittle him. Into this dynamic comes a German photographer whose motives are unclear, but her disruptive influence creates a power shift in the couple that will change them for good.

Negatives is a difficult film to sum up. It’s not a thriller, nor is it a dark comedy. It’s saucy at times but not erotic (despite being billed as such). As a character study, it keeps the viewer at a distance and makes the people you’re watching difficult to care about. It refuses to offer answers and remains obstinately ambiguous in its intentions. In other words, what the hell is it all about?

And yet, this 1968 debut from Peter Medak is a fascinating piece of British new wave – experimental, unconventional and hard to define.

Peter McEnery as Theo and Glenda Jackson in her first film role as Vivien are excellent, whether playing bored, frustrated, aroused or obsessed. Jackson, who just a year later would give an Oscar-winning performance in Ken Russell’s Women In Love, dominates the screen with her power, allowing you to glimpse why someone so awful might have been attractive to mild-mannered Theo in the first place.

But it’s when Reingard (Diane Cilento) moves into the spare room that things get really stirred up. Her fascination with the couple suggests a threesome’s on its way, but that’s not the thrust of her intentions. Her influence on Theo draws him away from his roleplaying as Crippen towards a new fantasy identity, one with more power, more charisma, more control – German officer Manfred von Richthofen, also known as the Red Baron.

This coincides with the death of his father, and as Theo’s mind begins to let fantasy matter more than reality, the tense power dynamic between him and Vivien shifts. Does Reingard want him for herself? In Negatives, nothing is that clear-cut.

Although it came first, Negatives is like the emotionless child of Women in Love and The Duke of Burgundy and, whilst both of those films are vastly superior, Negatives is still an intriguing puzzle.

Extras on this BFI release include a sweet interview with Medak, one with McEnery and a fascinating documentary about Crippen.