The pleasure here is in the fine detail, and Annan makes sure we feel every scrape of chisel on wood. The characterizations are not deep, but that's part of the genre.

It's the nervy camera, hugging the characters at hip height, the better to scrutinise each locked barrier to freedom, that most successfully builds the tension.

Almost everything that's enjoyable is a variation on stuff you've probably seen in superior prison movies, though Radcliffe's haunted performance is exceptionally compelling.