Dr. Calgary has been on a research trip on Antarctica for the last two years, but when he returns he learns that Jacko Argyle, the young hitch-hiking man he picked up on one of his last days in England, was accused of committing a murder and sentenced to prison where he later died. As Dr. Calgary could have alibied Jacko, he contacts the authorities to correct their mistake and visits Jacko's family to apologise. But their reaction is not what he had imagined...
I can understand why Christie liked this one, because this is a totally solid mystery plot with interesting characters. Dr. Calgary is a nice acquaintance, and his investigations are of the kind that you could imagine from a total novice in the detection business.
In my opinion, there are two problems with this novel, one big and one small. The smaller issue is simply that there is too little of the investigation business. We are treated to scenes with the different characters and also to the police force's somewhat reluctant re-opened case discussions, but quite a few of them simply serve as distractions from the main plot. This could, however, simply be down to this particular reader's tastes.
The larger problem, however, is one of those that puts a big hole in the plot and just about scuppers the whole thing - and that is the question why Jacko didn't reveal the true story as soon as he realised that he was going to be convicted of the crime. He knew what had happened, and while the rest of the world might not have believed him (though they should have, and obviously it was quite easy to get the true killer to reveal themselves, as we see in the dénouement here), it's simply not in accordance with what we learn of Jacko's character that he would keep quiet about it.
Unfortunately, that plot hole has to affect the rating of what is otherwise a pretty darn good mystery, and therefore I'll only give this a 51 out of 100. Please adjust your expectations accordingly if you don't feel that this admittedly peripheral plot hole will affect your enjoyment of the novel.