A second-order energy-preserving scheme is studied for the solution of the semilinear Cauchy Problem u(tt) - u(xx) - U(yy) + u3 = 0 (t > 0; x, y epsilon R). Smooth data functions of compact support are prescribed at t = 0. On any time interval [0, T], second-order convergence (up to logarithmic corrections) to the exact solution is established in both the energy and uniform norms.