The Plot device reference article from the English Wikipedia on 24-Apr-2004
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Plot device

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A plot device is a person or an object introduced to a story to affect or advance the plot of the story. In the hands of a skilled writer, the reader or viewer won't notice that the device is a construction of the author—it will seem to follow naturally from the setting or characters in the story. A poorly written story, on the other hand, may have such awkward or contrived plot devices that the reader has serious trouble maintaining suspension of disbelief; indeed, they may even leave plot holes.

Some plot devices include: