I can see why this would have been popular when it was written in the 1950s, but nowadays it mostly just seems quaint. This is especially so because DNA testing would render the entire story pointless in today's world of mysteries. Still, it's a worthwhile and fast read, good if you have anything like a Sunday reading plan. The story is set in Switzerland, which was probably the most interesting thing about it for me. I groove on those cultural and lifestyle tidbits. This is a different sort of mystery, where the focus is more on the cop who goes mad than on the resolution of the case itself. A little girl is found murdered, and the detective on the case makes a "pledge" (hence the title) to her parents that he will find the killer. He takes his promise a little too seriously as the years go by, and engages in some rather bizarre behavior to try to trap the culprit.