From the book:
Tommy Hambledon, back in England, was a gift that the intelligence department lost no time in using. He was an espionage agent par excellence with the additional qualifications of having been a practical police officer for many years. When one ship after another sank mysteriously not long out of Portsmouth harbor, Tommy was called on to find the source of the trouble. The case of the sinking ships turned into the case of the murdered MacGregor, and Mr. Hambledon of the Intelligence functioned as a police officer as well as a government agent.

His activities, accompanied by a running fire of his ironic, humorous comments on an England he had not know for twenty years, constitute a superlatively entertaining mystery story.