the trailer contains a massive spoiler, so here's the Morricone theme instead
"Everybody's got a right to be a sucker once."
It's the classic opener: the gunslinger stumbles upon a damsel in distress in the middle of the desert. This time the gunslinger is Clint Eastwood and the damsel is Shirley MacLaine. The two of them play Hogan and Sara. After Hogan guns down the group of would-be rapists, Sara puts her clothes back on.
Hogan's thrown for a loop when he sees the habit and the rosary. He doesn't feel right leaving a nun all alone in the desert, so he agrees to take her with him, even after he discovers Sara's in deep shit with the French for providing money and support to Mexican revolutionaries.
Two Mules for Sister Sara is a comedy that sometimes forgets it's also a western until it overcompensates in its climax, which is jarringly and uncharacteristically violent. The rest of the film is pretty funny, sure, but it must have been disappointing to see it during its original run, only a year after the release of Butch Cassidy and the Sundance Kid, which is really funny and a lot more evenly cooked.
The running gag: although she's a nun, Sara says and does some unlikely things. After Hogan helps her climb into a tree, he sincerely apologizes for touching her bottom. "It's no sin that you pushed me up the tree with your hands on my ass," she says. Hogan's double-take is priceless.
But that's pretty much all it is: funny. There's some amusing dialog, good writing, and a touching moment or two, but it's little more than a solid entertainment that feels like it's playing it a little too safe. It comes from a time when westerns were like Marvel movies and the studios were just as reluctant to adjust the formula as they are today. That so many people seem to consider Two Mules for Sister Sara to be some kind of classic sets the bar for classics just a little too low. It's a good movie and I'll probably even watch it again someday, but I personally wouldn't say it's great.
And that's just fine.