  In this Letter we study theoretically the interaction of optical waves in
nonlinear dynamical medium, i.e. medium with relaxation. Taking into account
the relaxation of the photoinduced nonlinearity we derive a single evolution
equation, namely the nonlinear Schr\"{o}dinger equation with coefficients
depending on parameters, for the case of degenerate two-wave mixing at the
reflection geometry in bulk Kerr-like medium possessing a nonlocal nonlinear
response. All coefficients of the single evolution equation for our system are
written out explicitly in terms of initial parameters. This is the first
analytical study of the evolution of nonlinear dynamical medium under the
action of two wave mixing; usually, it is studied numerically or experimentally
making use of some empirical assumptions. We briefly discuss various possible
scenarios of energy transport in the frame of the novel equation.
