The most fertile sub-genre of films about the '39-'45 war released over the following decade proved to be the special operation stories. This was a forerunner, and set a high standard, as well as establishing many of the conventions. The early scenes are mostly an explanation of what these units were.
It begins with the training of Belgian operatives by a support team of British boffins. Once primed, these ordinary men and women are parachuted into the resistance. The first half is episodic, as the spies are imbedded into the cause. This evolves into a single manoeuvre to release the underground leader from prison.
This cell is run by Simone Signoret, with Jack Warner and Gordon Jackson. There is immediately a huge shock as the relatable, avuncular Warner is uncovered as a German double agent and shot dead. The operation develops into a suspenseful action climax on the railways.
The cast of Ealing regulars is excellent, but Signoret elevates the film. The script goes into a lot of depth about the psychological trauma of spying, and the personal loss of these refugee heroes, who had to leave their families behind. It's film that acknowledges their astonishing bravery and incalculable sacrifice.