About MacGuffins, for both you and QueenNormaJean maybesnow?!:

It's not that the presence of the MacGuffin isn't critical to the plot. It's that the specific identity of the object in question isn't critical to the plot. You could swap it out for a host of other objects and still have the same plot.

A great example is Monty Python and the holy Grail. The plot is driven by King Arthur's desire to obtain the holy Grail. But is the story really about the holy grail? No, it's about the QUEST. He finds his Knights of the round table, they have adventures on the way, and randomly someone in modern times gets murdered and there's a police investigation. It could be the quest for the holy shroud, or the arc of the covenant, or it could be the pacifier of sir-not-appearing-in-this-film. It's a MacGuffin because it doesn't matter what the object is, but the quest for that object is most definitely critical to the plot. It matters that Arthur wants it, and that motivates him.

Cheers,
Michelle