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]