In this paper the dynamics for a third-order nonlinear difference equation is considered in detail. By utilizing some beautiful mathematical skills, we describe the rule for the trajectory structure of this equation clearly and completely. The successive lengths of positive and negative semicycles of any nontrivial solutions of this equation occur periodically with prime period 7; the rule is 3+, 2-, 1+, 1- in a period. Using the rule, we verify that the positive equilibrium point of the equation is globally asymptotically stable.