A MacGuffin is an object, or idea which serves adds no depth to a story but is nonetheless responsible for moving the narrative along from one point to another. Often, a MacGuffin is a valuable object which a character must acquire, protect, or transport. Although a MacGuffin is a type of plot device, not all plot devices are MacGuffins.
The nature of the MacGuffin itself doesn't matter -- it could be anything. The MacGuffin's only important attribute is its importance to the characters. It is the reason they take action, and its is the characters' actions -- not their motivations -- that are usually of most interest to the story's audience.
In Transformers fandom, a MacGuffin is sometimes referred to as an "Evil Invention / Alien Device", or "EI/AD". This term was coined by Kendrick Kerwin Chua to refer to the "machine of the week" style stories that were often told on the G1 cartoon.
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