We argue that the problem of adversarial plan recognition, where the observed agent actively tries to avoid detection, should be modeled in the game theoretic framework. We define the problem as an imperfect-information extensive-form game between the observer and the observed agent. We propose a novel algorithm that approximates the optimal solution in the game using Monte-Carlo sampling. The experimental evaluation is performed on a synthetic domain inspired by a network security problem. The proposed method produces significantly better results than several simple baselines on a practically large domain.