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Richard Hannay is introduced to Dr. Nils Larssen and his daughter Kirsten by Lord Hurst, the owner of a Chemicals business which operates out of Hurst Grange, a manor house in the English countryside.

Dr. Larssen is working on a serum which will produce a vaccine against 'the terrors of the earth' - typhus, tuberculosis and cholera, which may be needed in the event of germ warfare in any conflict with Germany. The idea is to produce vast amounts of the vaccine which Lord Hurst is prepared to back and then to market.

Also working in the laboratory at Hurst Grange is Edgar Voce, a German agent. Voce approaches Otto Von Schwabing, the murderous teutonic villain who is responsible for most, if not all of the treacherous activity of The Black Stone, the German Terrorist Organisation. Voce tells Von Schwabing that he is producing vaccine for German use in the event of warfare, and Von Schwabing keeps his ear to the ground.

Dr. Larssen becomes suspicious of Voce, and is poisoned by him, dying of what appears to be typhus in the local military hospital. Kirsten becomes suspicious of the caraffe of drinking water in her father's room, and tests it for traces of serum.

Voce leaves Hurst Grange to meet a sinister "clergyman" known as Dearth at a windmill which is also called 'The Sorrowful Mill'. In his possession he has a mysterious flask in a leather case. Richard Hannay follows him. Voce meets with an accident, and Hannay goes on to The Sorrowful Mill where he not only encounters Dearth, but also his arch-enemy, Von Schwabing. He is overpowered and locked in the Mill and in a bid to escape ends up on the mill blades.

Death tells Von Schwabing what the mysterious flask contains: It is, in Dearth's words "...a concentrate of the most powerful bacilli on earth, so deadly that a pint of it introduced into a city's water supply would kill everybody in that city within two days. And that's why our masters in the Unter Den Linden want it," he concludes to the sinister and brooding Von Schwabing.

In a gripping finale to this extremely fast moving and suspenseful episode of the series, Hannay must not only escape certain death himself, but must also rescue the occupants of London from certain death through an attempt to poison their water supply. Von Schwabing's behaviour on this occasion shocks even the usually unflappable Hannay.

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