In 228CE Hadrian's Wall in Britannia, and the brave auxiliary soldiers who man it, provide the physical security for the northern frontier of the mighty Roman Empire. However, only some enlightened few understand the graver supernatural threats lurking among the dark forests and trackless marshes of the border region. Those few are yourselves, the Brotherhood of Mithras, who serve among the auxiliaries.

But today you are just soldiers, as your century is part of a force that is responding to a call from the client kingdom north of the Wall. They are being threatened by an incursion of Caledonii from even farther north. The provincial governor himself is leading the mission, probably with a view to political advancement. You are just looking forward to getting out of your fort and doing some simple soldiering for once - surely, no supernatural horror could be involved this time?

The recently-opened Women's Library in London is a resource of learning and camaraderie for all - although some would maintain that it should not exist. Now, mysterious floods are damaging the building and the precious books held within. The ladies of Smythe's Discreet Investigations, London's first all-female detective agency, must find out what is happening before all is lost.