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Webster 1913 Edition

Noun.

A series of events in a play, proceeding collaterally with the main story, and subservient to it.

Dryden.

A clandestine scheme; a trick.

Addison.

, n.

A series of events in a play, proceeding collaterally with the main story, and subservient to it.

A clandestine scheme.

underplot ‎(plural underplots)

A subplot; a plot that is not the main plot of a story.
- 1882-89, A Collection Of Old English Plays, Vol. IV:
  I have not been able to discover the source of the very curious underplot of The Captives.
- 1906, Walter W. Greg, Pastoral Poetry and Pastoral Drama:
  Thus, again to quote Mr. Homer Smith, it has been held that 'In general the pastoral incidents serve as an underplot, utterly foreign in spirit to the main plot.'

subplot — see subplot