Maguffin

-A maguffin (sometimes known as a mcguffin or macguffin) is a plot device that motivates the character and advances the story, but has little other relevance to the story.

Hitchcock's quote-"We have a name in the studio, and we call it the 'macguffin'. It is the mechanical element that usually crops up in any story. In crook stories it is always the necklace and in spy stories it is always the papers."