A MacGuffin is an object or idea which 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. For example, Metroplex's transformation cog is a MacGuffin. 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. Whatever it is, the MacGuffin will usually not be fully integrated into the story: typically the exact nature of its powers or origin, the reason why it never influenced previous story events, or the reason why it and it alone is so crucial to its alleged role, will never be fully explained; to return to the prior example, there was no prior or subsequent mention of Metroplex' transformation cog, or anyone else's, nor any clear reason given why the Autobots had to retrieve the original instead of building a replacement. The MacGuffin's only important attribute is its importance to the characters. It is the reason they take action, and it 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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