Being a tramp is not always a disadvantage. For example when there is a masquerade ball. Indeed wearing flea-ridden rags might just mean having donned a costume. This is what happens to two resourceful Brussels bums, La Cloche and Picolard, who manage to gain entry in a fancy-dress ball. Once there, La Cloche is mistaken by an oriental prince for a respected doctor. His mission will be to give care to a music-hall diva. Even more exciting, he is asked to vaccinate a whole troupe of showgirls. Trouble guaranteed.

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.