Coined by Alfred Hitchcock, a MacGuffin
(sometimes McGuffin or maguffin)
is "a plot element that catches the viewers' attention or drives the
plot of a work of fiction".
The defining aspect of a MacGuffin is that the major players in the
story are (at least initially) willing to do and sacrifice almost
anything to obtain it, regardless of what the MacGuffin actually is. In
fact, the specific nature of the MacGuffin may be ambiguous, undefined,
generic, left open to interpretation or otherwise completely
unimportant to the plot. Common examples are money, victory, glory,
survival, a source of power, or a potential threat, or it may simply be
something entirely unexplained.
The MacGuffin is common in films, especially thrillers.
Usually, though not always, the MacGuffin is the central focus of the
film in the first act, and then
declines in importance as the struggles and motivations of characters
play out. It may come back into play at the climax of the story, but
sometimes the MacGuffin is actually forgotten by the end of the film.
Multiple MacGuffins are sometimes—somewhat
derisively—referred to as plot coupons.
(from wikipedia, the free encyclopedia)
"In crook stories
it is almost always the necklace,
in spy stories it is most always the papers."