Plot planning is one of the key steps in automatic generation of computer animation, aiming at converting the abstract plots of a story into a series of concrete actions. This paper presents the architecture of plot planning model, in which we employ an agent-based method to develop the abstract plot. In our agent hierarchy, there are two kinds of agents: drama agent and plot agent, both of which can develop the abstract plot according to their knowledge and strategies. Furthermore, to avoid the irrationality of the results, we address a validity checking mechanism to ensure the consistency of the concrete actions.