How we change what others think, feel, believe and do
The MaGuffin is an object, event or 'thing' that is usually of central focus in the first act, but is ultimately unimportant to the plot.
Although the item is unimportant, it provides motivation that kicks things into action. Later MacGuffins may be used to prevent things flagging and keep the story moving on.
In a detective story, something 'important' goes missing. What it is does not really matter - the plot is more about finding the criminal.
Something important must be delivered. The contents are, however, not important.
Someone is killed. Who it is is not significant, but it is useful to introduce characters and kick off the real action.
MacGuffins are often fairly ridiculous, particularly when viewed in a logical or scientific light, yet many people accept their viability within the story, which perhaps shows the power of suspending disbelief. Perhaps a part of its purpose is to create that suspension, triggering the audience into a state where they will believe anything.
The term was originated by Alfred Hitchcock who, in 1939, said:
"In regard to the tune [espionage intelligence encoded in music], 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."
The MacGuffin is a kind of reverse Chekhov's Gun in its early significance and later insignificance.