Vicente Amorim’s and John Wrathall’s adaptation of CP Taylor’s play is essentially a parable about human weakness. What would you do, it asks, if a questionable dictator admired your work and sent an emissary to persuade you with much subtle praise, and a threat or two, to join his cause?

Would you be flattered into submission like John Halder (Viggo Mortensen), who, in doing a small favour to avoid trouble in Hitler’s Reich, found himself wading deeper and deeper into the Nazi morass?

Halder, a German professor of literature, has good reason to be beholden to the regime. While others’ books are burned, his half-forgotten novel on compassionate euthanasia is taken up in support of government propaganda. He falls for a student (Jodie Whittaker), who believes in the optimistic current of nationalism that Hitler induced in the early Thirties, drifts away from his wife (Anastasia Hille) and finds his circumstances greatly improved.

He may be foolish but he is not an evil man. He simply hopes for the best and shades his eyes from the truth — and is thus unaware of what he is doing, as Taylor believed many thousands of others were. Only the eventual fate of his Jewish best friend (Jason Isaacs) makes him see the light.

The original play, once dubbed one of the 100 best of the century, is fleshed out with skill by Amorim but somehow his film never comes fully to life. Even its melodramatic ending falls flat. Whether it is because Mortensen, a very good actor, is fundamentally unsuited to this kind of role, or because the argument develops too obviously and too slowly, the whole lacks real conviction — oddly, like Halder himself.