Psychic Nan Killian and medium Sarah Lyon-White, and their clever birds, the raven Neville and the parrot Grey, have been agents of the Elemental Fire Master, Lord Alderscroft, since they left school. Now they have a new commission: to work with the famous man living at 221 Baker Street. But not the man living in flat B; they are to assist the one living in flat C. Taking the occult cases his more famous friend refuses, Doctor John Watson and his wife Mary, who are themselves Elemental Masters of Water and Air, will need every skill the girls and their birds can muster.

Together, the girls will confront the mysterious and deadly entity that nearly killed them as children: the infamous Haunt of Number 10 Berkeley Square, and a multitude of spirits that threaten to destroy not only a lifelong friendship, but far more than that.