During the events of A Study in Sable, Nan, a psychic, and Sarah, a medium, left the school that helped them hone their abilities to go to London where they met Holmes and the Watsons. Their purpose, as directed by the Wizard of London, Lord Alderscroft, was to help the Watsons, each an elemental master in their own right, as they help solve the more supernatural cases that Holmes tends to ignore. While the troubles that the girls faced in A Study in Sable focused heavily on Sarah's mediumistic abilities, the nature of the threat in A Scandal in Battersea will test Nan's skills.
In the London district of Battersea, a wealthy scion living far below his means finds an old hand-written tome that explains how to summon a dark creature with a great reward as payment for the thing's release. All it needs is a person with some magical aptitude and little in the way of scruples. It finds both in Alexandre Harcourt, especially since his servent, Alf, has had a few magicians as masters in the past and knows how to get the riskier items the creature needs - people.
As Alexandre and Alf start their rituals, they find the thing they are summoning craves pairs of people. While both get sucked into the void in the basement, only one comes out, and it is a mindless and soulless husk of a person that is only able to respond to basic commands. It's clear the creature has some plan for the returned person though because it commands Alexandre to let them loose so that they can be picked up and cared for. Unfortunately, for Alexandre, the slowly growing number of victims has caught the attention of Holmes, the Watsons, Nan and Sarah.
At about the same time that the wandering women are being brought in from the cold, snow-covered streets, Nan and Sarah are asked to look into a case at a mental hospital. At first glance, it appears the girl was checked into the hospital because she was suffering from an illness, but Nan quickly identifies the girl as having precognitive abilities and she is actually having visions. It isn't long before the group realizes that the girl's visions are tied to the soulless victims and, with the help of other friends like Pan, as well as Memsa'b and Sahib (Nan and Sarah's former teachers), they start to piece together the nature of the growing threat.
Much like A Study in Sable, A Scandal in Battersea does feature Sherlock Holmes as a main character, but not nearly as much as some descriptions of the book might suggest. Holmes is a more prominent character in this story than the previous, but his purpose seems to still be that of a character looking for anything but the supernatural cause for the problems at hand. The Holmes of this world still feels a lot like the classical character, so his stubbornness in the face of apparent superstition makes sense until he has overwhelming evidence to the contrary that forces him to reassess many of his preconceived notions. While he does eventually concede to several aspects about the mystic world around him, he still finds many of the aspects that the other characters in the book take for granted as fiction. But, the character is growing, he just remains a smaller part of the book than you might expect ahead of time. The focus is very much on Nan and Sarah (the former more than the latter). It will be interesting to see how much more Lackey invests in this pair given that most of her other Elemental Masters books are one-off pieces that aren't nearly as connected as The Wizard of London, A Study in Sable and A Scandal in Battersea. Regardless, this was a fun read, but not nearly as stand-alone as other Elemental Masters novels, so you might want to pick up the previous books before jumping into this one.