Isabelle Eberhardt was one of those Victorian-era women who did not so much throw over the traces as fling them to the heavens, then run away laughing. Born in Switzerland, to an eccentric, drunken anarchist father and a inept German mother who had fled to him from her Russian general husband, Isabelle grew up, at least on the account of New Anatomies, the play that has just opened in London, addicted to fantasies of the desert and Islam that had been her childhood refuge.

As a young woman, with both parents dead, she visited, with her staunchly conventional sister Natalie, her beloved brother Antoine in Algiers, where he had run away to the Foreign Legion. There she adopted male dress, the name Si Mahmoud, and took to the desert in the company of members of a Sufi mystic order. Despite her Islamic faith, she also, said one acquaintance, “drank more than a Legionnaire, smoked more kif than a hashish addict, and made love for the love of making love”.

Forced back to Paris, she can only return to the desert as a French spy. But can she be truly free? Somehow you know what the answer is going to be, even if you haven’t cheated by reading the biography first.

Timberlake Wertenbaker’s play sticks closely to Isabelle’s life story, rather too closely; it sometimes feels that you are listening to a lecture as the script breaks that cardinal rule of “show don’t tell”. There are also some heavy-handed political messages about the virtues of Islam and the life of the desert, as opposed to the Christian values of the Europeans.

Nonetheless, in its better scenes there are gorgeous flights of fancy, as Isabelle lets sand trickle through her fingers, referring to it as “the inside of water”, and when she describes herself as “the pervert seed that would not flourish in manure”. There are also plenty of humorous moments, as when Natalie – devoted wife of a shopkeeper, tells Isabelle that the way find find a husband is to be seen dusting enthusiastically in that very shop.

The director, Antony Law, has made an interesting job of the staging – the use of sand, so central to Isabelle’s vision – is inventive, and the lighting through the fine screen of tent cotton, is effective.

The acting is, however, uneven. Madeleine Lund is powerful, if somewhat uni-dimensional, as Isabelle; we meet her as a rebellious, difficult, drug and drink-addicted character at the end of her life in the opening scene, and this seems also to be the young Isabelle who first arrives in Algiers.

The team of five other actresses does a good job with the female parts. Hannah Khalil is the repressed voice of conventionality in Isabelle’s sister Natalie, Annette Roche is the cross-dressing journalist who tried rather helplessly to help the increasingly out-of-control Isabelle as her life gets more and more complicated – shocked that her overtly lesbian lifestyle suddenly looks almost respectable.

Where the cast flounders is in the male parts – as French military figures, and desert locals, they just don’t change enough, in voice, in mannerism, in look, from their female roles.

This is a play with a lot of potential – a fantastic story. You’d just love to take a knife to the script to cut away the preaching and the exposition, and to enlarge the cast, then it could be really something. Even now, it’s an interesting evening; it just could be a lot more.

The play is at the Cockpit Theatre in Marylebone tomorrow (Saturday) at 3.15pm and 7.45pm, then moves to the Hen & Chickens in Islington from February 21 to March 4.