O Jerusalem marks the fifth appearance of Mary Russell and Sherlock Holmes. This time around they have fled to Palestine on a mission for Mycroft Holmes. Disguised as itinerant Muslims and paired with two Arab spies, Russell and Holmes travel through the Holy Land trying to figure out exactly why Mycroft has sent them. A pair of seemingly unrelated murders sets them on the track of a brilliant and power-hungry killer. Only Holmes and Russell (along with some unexpected allies) can stop their adversary from destroying Jerusalem. If they can get to him in time.