It's always hard to review a spy movie without giving away some of the twists and turns, but Amazon's own editorial - "From the back Cover" gives away a big spoiler, and I'm glad I hadn't read it before seeing the movie !
This movie is actually available on DVD in Australia, though there are no special features and the print is quite scratchy with a few clumsy editing cuts.
I could also swear that some of the actors' lines have been revoiced, though this is not really a big issue for such an old movie.
Certain parts of the movie have a flurry of fast-cuts, enough to rival your average MTV music clip.
Other reviewers have covered the plot well enough, so I'll just say that the first half is an interesting, fast-paced, intricately plotted spy yarn.
Unfortunately the second half is just a simple sea drama. The movie tries to end with a big action sequence but the low budget and pedestrian direction takes the story out with a whimper rather than a bang. The earlier emphasis on characters and motivations evaporates into uninspired action scenes which leave the viewer with no-one and nothing to care about.
And as for my review title , perhaps I'm misinterpreting something, but the spy in black wasn't a spy at all. He just seemed to be a high-ranking courier for the information gained by the real spy. But I guess "Courier in Black" lacks punch !