n. A subplot; a plot that is not the main plot of a story.
the GNU version of the Collaborative International Dictionary of English
n. A series of events in a play, proceeding collaterally with the main story, and subservient to it.
n. A clandestine scheme; a trick.
The Century Dictionary and Cyclopedia
n. A plot subordinate to another plot, as in a play or a novel.
n. An underhand scheme; a trick.
Word Usage
"In other respects, however, it is certainly a great advance on its predecessors, especially in its underplot, which is for the first time connected satisfactorily with the main argument."