The idea of "Francis" is clearly ridiculous and cannot possibly work. After all, Francis is a mule that talks!!! Amazingly, while a thoroughly ridiculous idea, Universal Studios managed to make it work for two reasons. First, Chill Wills' voice and dialog was perfect as Francis. Second, the writing was terrific and they made the very most of what material they were given.
The story is told in a flashback, as Peter (Donald O'Connor) tells his boss about his war exploits. During this long story, Peter recounts how he discovered a talking mule while on duty in the Pacific during WWII...and Francis not only can talk, but he's a heck of a lot smarter than Peter! Using Francis' help, Peter manages to impress his commander when again and again, he manages to know exactly what the Japanese are going to do next! But Peter makes the huge mistake of telling them where he got in the information...and they naturally think he's flipped his lid! What's next? Well, see the film.
Whether or not you enjoy "Francis" depends on whether you see it in the first place. I think the concept seems so goofy that most folks wouldn't try the film on a dare! Well, I dare you to try it...and don't be surprised if, like me, you really like it!