When Gérard Philipe’s “second rate devil” Mephistopheles makes that famous pact with Michel Simon’s forlorn alchemist Faust — one soul in return for sex, power, and youth, not to mention the secrets of the universe — the two exchange bodies, with the ancient Faust transformed into the handsome, dashing young Philipe, and vice versa. But then Philipe/Faust gets a peek at the post-atomic horrors to come…