This book has the most tragic beginning among all the stories in the series. One of the closest Wolfe's friends is killed and the police is completely baffled in the murder investigation. The famous New York detective makes exactly the same progress in his own investigation as the police: none. He gets more and more frustrated and finally makes the unthinkable: not only he leaves his brown house, he also leaves US and goes to Montenegro (The Black Mountain) as the only clue he found leads him to his land of birth. His faithful sidekick Archie Goodwin goes with him posing as his son.
The book has more serious undertones than a typical Nero Wolfe story as the majority of it is spent in Montenegro which according to How Rex Stout saw it was not a nice place - I really have no idea whether it was really as bad. Archie does not speak the local language(s), so he has much less opportunities to practice his trademarked wisecracking which really adds humor to the series. On the other hand, this is one of the few books where Nero Wolfe is forced to do his own legwork as opposed to delegating it to the usual cast of operatives which really makes this book stand up among the crowd.
Another thing of note: finding the culprit was never about revenge for Nero Wolfe, it was always about bringing the murderer to justice. This was in sharp contrast to how they used to do things in Montenegro; again I am talking about Montenegro envisioned by Rex Stout: it is hard to say how right he was without actually being in the country during that time.
The mystery was fairly weak with Nero Wolfe not doing practically any mental efforts at all: all the information was readily available once the detectives arrive to the right place.
One of the few books in the series where the laziest genius detective had to do his own legwork gets 3.5 stars.