Book #3 in what is sometimes considered the Inspector Marshall series.
Supposedly based on actual events, this tells of the adventure where Captain Newman learns of a plot by "an international association of students" to assassinate Italian dictator Mussolini in 1935. Not wanting to see the fascist leader become a martyr to his cause, Newman works with Inspector Marshall to save the man's life.
Note: this is the first reference to Special Branch as being where Marshall hangs his official hat.