This is Ambler writing for older, more contemplative readers. (Ambler for grown-ups, you might say.) Yes it's slower than many of his other books; it lacks the big action-payoff climax so many of his other books have. But it more than makes up for the deficit, with vivid atmosphere, engaging characters, and gradual, long-term buildup of tension. Ambler manages this so masterfully that I don't miss the big heart-pumping moments from his other books. (Adrenalin is, after all, generic. If you can't get off without it, well, you can always read something by Lee Child or Thomas Harris or someone like that, immediately after you finish with this book.)
The title-character is a cynical doctor with a wry, sardonic sense of humor, and he writes with a doctor's understated, faux-clinical narrative style. (Besides being the protagonist, he's also the first-person narrator.) His unappealing personality makes him all the more appealing to the reader. (This is true of many of the characters in Ambler's best books, such as Arthur Simpson in The Light of Day and Dirty Story, and almost everyone in Passage of Arms.)
"Doctor Frigo" is a nickname which his underlings and colleagues call him behind his back, because of his cold, clinical manner. He has a weird girlfriend who thinks of everything in terms of the history of her family (she's descended from the Bourbon dynasty). And his father was a charismatic leader in a third-world country, until he got assassinated. Now, years later, Dr. Frigo has a new patient, another political leader, who appears to have had something to do with the assassination - maybe. As the book progresses, it gradually focuses more and more intensely on three questions: will Dr. Frigo take vengeance against the assassins? Will he return to his father's homeland and lead a revolutionary movement there, whose members his family name would inspire? And, will he be able to avoid being assassinated himself long enough to decide?