Born in Châteaubriant, Loire-Inférieure (now Loire-Atlantique), France, Chaplais was educated at the Collège Saint-Sauveur in Redon and the University of Rennes, where he studied Law and Classics, with a view of becoming an academic lawyer. His education was interrupted by World War II, during which he served with the French Army until the 1940 Armistice. A member of the French Resistance, Chaplais was captured by the Gestapo and sent to Buchenwald. For his wartime activities he received the "Médaille de la Résistance".