The movie was made in 1955, just ten years after the end of the Second World War. It's about the development and deployment of the bouncing bombs used against the German hydroelectric dams in the Ruhr Valley (apparently called "Operation Chastise). You would expect that it being made so shortly after the war would mean Rah-Rah patriotism, but not as much as I expected. The opening credits expressed gratitude to all the crew and surviving relatives who helped with the film - and this is one of the places the film falls down: everyone who worked on the process was JUST LOVELY. Not a bad or incompetent person to be seen anywhere.
The movie opens with Barnes Wallis (played by Michael Redgrave) working on the design of the bomb, and the first half of the movie is about that and the politics of getting practical trials for the bomb. I found that fascinating - as were the incredibly simple, but very effective, bomb sight that was designed specifically for the mission, and their method for staying at a very exact height on the approach. But around the half way mark, we switch to the actual bombing mission: we watch the men chatting, waiting for the call. We watch them going to the planes. We watch them board the planes, taxi, take off, fly over the ocean, fly over the land. It got somewhat more interesting with the actual bomb runs, but even that was too long. At the end, we're informed of the success of the runs against the Möhne and Edersee Dams, but the Sorpe Dam - which had previously been mentioned several times - isn't mentioned at all. Apparently because it wasn't damaged, but just forgetting to mention that - weird.
I suspect the historical accuracy of the technology is excellent. And they flew four(?) Avro Lancasters simultaneously in the film, a thing we'll never see again (there's one flying in the entire world in 2015). So from a technical point of view it's a wonderful movie. Dramatically, not so much. Recommended for war buffs, but not for the drama or action.