The Two Mistletoe Berries

First printed in the UK magazine called The Royal Magazine #218, December 1916. Collected in Donovan of Whitehall.
"[In] the following story, [Donovan] penetrated into Belgrade and obtained information of enormous value to the Allies." The mistletoe berries in the title are used to created a particularly nasty concoction which is the tool needed by Donovan to learn exactly what the Austrians were up with in Serbia.