We present a computational cognitive model of mind wandering, an important cognitive phenomenon whose mechanisms are involved in insight, problem-solving, and creativity. The model posits that mind wandering begins when one is not engaged in goal-oriented cognition, whether when between tasks or when in the middle of a task but not actively thinking about one's goal. At such times, the model thinks about other, highly-activated thoughts in memory. This model sheds light on both how task-oriented and more basic cognitive processes interact, as well as how mind wandering content is generated; both unresolved questions for mind wandering research. We compare our model against data presented by McVay & Kane (2013), who induced mind wandering in a laboratory setting by embedding participants' personal goals and concerns in a lexical SART task. Overall, our model matched the data's mind wandering rates very well. We discuss implications and future work on the model.