The Collaborative International Dictionary of English v.0.48:

Counterplot \Coun`ter*plot"\ (koun`t?r-pl?t"), v. t. [imp. & p.
   p. Counterplotted; p. pr. & vb. n. Counterplotting.]
   To oppose, as another plot, by plotting; to attempt to
   frustrate, as a stratagem, by stratagem.
   [1913 Webster]

         Every wile had proved abortive, every plot had been
         counterplotted.                          --De Quinsey.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Counterplot \Coun"ter*plot`\ (koun"t[~e]r-pl?t`), n.
   A plot or artifice opposed to another. --L'Estrange.
   [1913 Webster]

WordNet (r) 3.0 (2006):

counterplot
    n 1: a plot intended to subvert another plot [syn:
         counterplot, counterplan]
    v 1: make a plot in response to another plot