Based on "Peril at End House" by Agatha Christie.
When Madeleine tells "Victor" (actually Emile posing incognito as the new handyman) to hang up the lanterns in the yard for the birthday party, she unknowingly uses his real surname - the French word for lantern is 'lampion'.
In both this episode and the previous, "La Plume Empoisonnée", a character screams in Lampion's face "JE NE L'AI PAS TUÉ!" ("I didn't kill her!")