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.'