Robert Bresson's films are about people trying to find their way into some sort-of grace or salvation. About dealing with the mystical or god-like in everyday situations and often through dirty or unpleasant means. In this film we find a member of the French Resistance (based on André Devigny) being housed in Montluc Prison while awaiting to be executed during WWII. The fighter (called Fontaine in the movie) is not content to wait for his death and immediately plans to escape, but the Nazis are not about to make it that easy. He now has to make it out of this heavily-fortified prison alive with every move under close watch.
This film is an A to Z of what one gets in a Bresson film. Anonymous actors (who Bresson always referred to as his models) do not act so much as recite the lines with as little emotion as possible. There is a "holy minimalism" in how the action is portrayed in these films (and yes, despite how restrained the actors "models" are, they're the ones that drive the plot). The sparse use of music is another thing that distinguishes this and other movies of Bresson, because of how precise he uses it is use. We get some music during the movie's intro and then none for about 30 minutes until a random scene where the prisoners are emptying their slop jars in the prison yards. He said that he wanted this to represent a precise moment of ritual for the characters lives. This all goes back to Bresson's quest to purge cinema or movies (which Bresson unhelpfully calls "cinematography") of as much of the influence of the theater or plays as possible. That's why he calls his movie actors "models": in his mind actors belong in places like Broadway, film has to have its own language distinct from what came before it. Other directors like Andrei Tarkovsky and Mohsen Makhmalbaf thought on similar lines, but neither took it as far as Bresson to try and remake the basic language and function and cinema as we know it. Even his devoted fanbase of film-makers in the French New Wave were not prepared to "transgress" (though Bresson would probably see it as a purification) in the way that he was.
This film, and all of Bresson's films, is an experience unto itself and you either going to like it or hate it. This is a film about prisoners escaping the Nazis, there were countless films with that plot before A Man Escaped, and have been countless since, but I guarantee none of them look like this.