King Of Hearts(1966)
An ornithologist mistaken for an explosives expert is sent alone into a small French town during WWI to investigate a garbled report from the resistance about a bomb which the departing Germans have set to blow up a weapons cache. He arrives to find a very eccentric group of townspeople, inmates of the local insane asylum, as it turns out, who have stepped into the characters of the fleeing villagers.
This is a Movie Channel promo for the movie.
This is an HBO promo for the movie.