While the Blitz bombards London, the boisterous grandchildren of Sir Richard March have descended upon Swanswater Manor in Kent for a family gathering and the finalising of the patriarch's will. Disgruntled by the behaviour and life choices of his heirs, March seems poised to deny all of them their inheritance and heads out to his lodge to make arrangements — only to be discovered dead the next morning with strychnine in his blood.

With evidence at the crime scene suggesting that nobody could possibly have entered the lodge to murder March, Inspector Cockrill— the "Terror of Kent"— has the challenge of finding any plausible solution for this impossible crime before death comes to darken the doors of Swanswater once more.