Eddie Izzard, comedy genius and heroic campaigner for the European cause, is bizarrely miscast in a deadpan-serious acting role for this weird, strained, second world war spy melodrama inspired by a stranger-than-fiction true story. The transgender star switches back here to what Izzard has playfully called “boy mode” for a boy’s-own-story in the style of John Buchan or Erskine Childers, or maybe one of Michael Palin’s Ripping Yarns, though with the jokes and comic self-awareness systematically removed. It’s a muddled, unrelaxed tale from Izzard as co-writer and co-producer, whose dramatic gears keep slipping, and which is never entirely sure where our sympathies should lie.

Izzard plays Thomas Miller, a teacher of Anglo-German ancestry who in the fraught summer of 1939 takes a job at the Augusta Victoria college in Bexhill-on-Sea, a kind of genteel finishing school for expatriate German adolescent girls; he is interviewed for the job by the naively pro-German English headmistress Miss Rocholl (with Judi Dench sleepwalking through the role). This extraordinary school really did exist throughout the 1930s, with a swastika discreetly sewn into its crest next to the union jack, and it was attended by the daughters and god-daughters of some very senior Nazis. Izzard imagines what might happen if, as war loomed, this place was the centre of a Nazi intelligence plot.

A lively idea for a drama, but the sheer oddity of the real-life premise slows it down, and the film is unsure about whether we should see these teen blonde Mädchen pupils as Midwich-Cuckoo mini-Nazis or as poignant dupes. A spy story depends on secrecy and concealment, but this school is entirely open about its pro-German and pro-Nazi leanings from the outset. As for the character of Miller, he is supposed to be a real action hero, and there are loads of shots of Izzard running athletically and surreally away from the camera across open fields. The film itself runs out of breath.

• Six Minutes to Midnight is released on 26 March on Sky Cinema.