When last we saw Nikola he had gotten the Chinese Prayer stick. In Dr Nikola, we discover that he uses it to gain entry to the secret society known as The Three.
In the story he hires Wilfred Bruce, an out of work Englishman in China, to assist him in making their way through China disguised as Chinese men. In the first book Nikola was clearly a villain, kidnapping young women for the ransom of the Chinese stick, in this book Nikola is more of an adventurer and dedicated scientist.
In 1896, an Englishman posing as Chinese was acceptable (I seem to recall hearing tales of Richard Burton disguising himself as an Arab to explore that part of the world) but to modern eyes it comes off as wrong. I'd be intrigued to see how well received the next Mission: Impossible movie would be if Tom Cruise's Ethan Hunt disguises himself as a Korean to infiltrate North Korea to find some McGuffin.
This felt like a really long book, it took Nikola and Bruce a long time to get to the monastery where the Three reside. Once there Nikola gains some of their secret knowledge including a new aesthetic. Then they are discovered to be imposters and they have to run for the final part of the novel.
I prefer Nikola as an honourable villain, while it was interesting to see Nikola in a different light this book wasn't very satisfying. I wonder what plans Nikola has for his ill-gotten knowledge