In Tufnell Park, North London, a set of railway tracks run under a school playground, leading to and from King's Cross. Wet, filthy, dangerous. Lovely place. And one Sunday before Christmas, Abigail Kamara, one of my endless brood of cousins, dragged me and my long-suffering colleague Lesley May down there to look for a ghost.

We found one.

And that was that, I thought come Monday. First case of the day: Person Unknown has been stabbed to death on the tracks at Baker Street Underground. Magic may have been involved. Sure enough, the weapon turns out to be saturated in the tell-tale traces left by magic.

But Person Unknown turns out to be the son of a US senator, so before you can say "international incident", FBI agent Kimberley Reynolds and her firmly held religious beliefs are on my case.

And down in the dark, in the Tube tunnels of London, along with the buried rivers and remnants of Victorian sewer systems, I'm hearing some really strange things...