The Secret of 421 (1938)

Alphonse and his wife Zulma hold the 421, a tavern in the outskirts of Lille, in the North of France. It is always full of regulars who play cards there and drink gin and exchange words in ch'timi, the local patois. But there has been trouble lately: felonies have been committed in the surroundings. So who can be those two strangers who have just rented a room each at the 421? Each sets about doing a bit of detective work.