King's time-line gets kinda screwy around this point in the series. Book five, O Jerusalem, backtracks to where the series started with the plot taking place in the middle of The Beekeeper's Apprentice. At the time I didn't feel like doing a flashback with the characters, so I skipped to this book since it takes place almost immediately after The Moor. I had assumed that I would be safe skipping O Jerusalem but I was sort of wrong.
During The Beekeeper's Apprentice, Mary and Holmes went on a trip to Jerusalem on some business for Holmes' brother Mycroft. During this trip they made friends with two brothers, Ali and Mahmoud, one of which turns up bleeding on their doorstep. The injured Ali, has come to ask for Holmes and Mary's help in convincing his brother to return to Jerusalem with him. Owing Ali and Mahmoud their loyalty for the help they offered all those years ago, Holmes and Mary set on a journey that both of them view as rather pointless. However, when they arrive at the sprawling mansion that Mahmoud has taken up residence in they are disturbed by the immense changes in their friend. Shackled with an outdated responsibility to his family, Mahmoud has become quite the miserable drunk. Wanting to help, Mary and Holmes attempt to figure out a way to make it so Mahmoud can return to Jerusalem with Ali.
The story here had a really interesting tone. This is the first novel in the series where King gives us a glimpse of the roaring 20's the way I typically imagine it, with extravagant parties and a cast of eclectic characters. It was pretty amusing to see serious Mary navigate her way through the fast-paced party atmosphere that this investigation foisted on her. Her horror/amusement at the parties and people found in them, paired with Holmes dodging out of going to the shindigs with her, was a definite highlight. It also offered a nice contrast to the more depressing aspects of the mystery that involved a World War I execution.
Seriously though, I was a little lost in parts of this novel because I hadn't read O Jerusalem. Which took me by surprise since most of King's novels seem to stand well on their own. However, King does give some back-story concerning Holmes and Mary's history with the two brothers, but it wasn't enough to explain why they both have such unswerving loyalty to Mahmoud and Ali. All this, of course, is my own fault for skipping the previous book. So I would definitely recommend reading O Jerusalem before diving into this one.