An Invisible Murder

When travelling cook Jenny Starling starts her new job for the aristocracy living in a genuine castle, she is thrilled. She envisions nothing more arduous than days spent preparing her beloved recipes. When a fabulous jewelled dagger, one of the castle's many art treasures, is used to murder a member of the staff and the Lady of the house insists that Jenny - quite literally - helps the police with their enquiries, it seems as if the reluctant sleuth must once again discover the identity of the killer amongst them. But how was it done, when the murder was committed in front of several impeccable witnesses - none of whom saw a thing?