Sometimes a film will surprise you, and be a marvellous watch. On the surface Professor Marston & the Wonder Women didn't seem like much; simply well-timed exploitation of a hit film. Yet this is not the film that has reached us. Instead we have a touching exploration of a polygamous relationship, a necessary story from a time when lesbianism was considered a mental illness in the United States.

The Professor Marston of the title is Dr William Marston, a Harvard psychologist, inventor of an early version of the lie detector and, later in his career, the creator of Wonder Woman. The Wonder Women are his wife, Elizabeth Marston, and their lover, Olive Byrne. The film explores their relationship through raising children together, career setbacks and societal rejection.

Writer-director Angela Robinson brings a welcome lightness of touch here, comfortably managing the narrative transitions that are central to the film. The story is played straight, with no need to wink to the audience. It doesn't matter if you have any interest in Wonder Woman to enjoy this film. The comic merely acts as the Macguffin, driving the film forward in the second half of the movie and providing a framing device. It is the more intimate moments that the film is at its best, separate from the Amazonian Princess you would think this film would be about.

Professor Marston & the Wonder Women is aided by a strong trio at its centre. Luke Evans gives the Professor a welcome swagger but is also more then capable of handling the dramatic moments. Rebecca Hall is exceptionally good as the Mrs of the marriage, her Elizabeth a spiky creation and dominant force in the film. And finally Bella Heathcote is great as the third part of the relationship, developing throughout the film. Their relationship is approached in a refreshingly warm way, with the impact to them of far greater importance than any kind of backlash. There is a strong chemistry between the leads that lights up the screen, whether it is scenes where they explore their sexual side or discuss the findings of Jung. And it always helps when Oliver Platt (who cameos as the founder of Educational Comics) is on screen: he provides some good comic relief here.

This film surprised me, finding a route into a curious story that has produced an endearingly charming, surprisingly touching biopic. The lightness of touch elevates proceedings, aided by the great lead performances. Wonder Woman has two great films about her in one year.

This is a London Film Festival preview and Professor Marston & the Wonder Women will be released on Friday 10th November.