OK, but that's just occupying the Vatican, kidnapping the Pope and stealing some art, right? All of which fits the Nazi profile we've come to know and loathe. But there was a second component to the plan. Once the first wave of soldiers secured the Vatican and got their hands on Pius, a second, secret group would come in under the pretense of rescuing the Pope, kill the first group under the pretense that the guys in the first group were really Italian assassins, then accidentally shoot the Pope in the chaotic melee that followed. But it would be cool, because the Nazis could blame the Italians for the gaffe when it was all over. What could possibly go wrong?
Fortunately, the plot never took place because one of the inside men alerted the Italians before it ever got underway. The craziest part is that the scheme even got past the "What if we took over the Vatican?" phase in the first place. According to historian Robert Katz, assassinating Pope Pius XII posed zero potential benefits to the Axis powers, and probably would have ushered in a global backlash that would have made "the Ten Plagues that rained down on the pharaoh ... look like confetti." Which is a hard thing to pull off, if you think about it.
You have to hand it to the Nazis. It takes hard work to be that consistently terrible.