In the latter days of the First World War

Poirot and Hastings meet Captain Vincent Lemesurier and his uncle Hugo, a chemist. Cousin Roger rushes in with the news that Vincent's father has had a serious fall from a horse and is not expected to last the night. Vincent, the eldest and only of three sons to survive the war, has a strong reaction to the news -- partly due to the Lemesurier curse. No first born son has lived to inherit the family estate since the Middle Ages...