People need to critically comprehend information across multiple sources that express contradictory viewpoints to make decisions on relevant everyday-life issues and participate in the democratic discourse. However, the processing of multiple documents depends on readers' prior beliefs. The present study investigated the moderating effect of prompting planning on the link between prior beliefs and multiple-documents comprehension. Eighty university students participated in the study. First, their prior beliefs, prior knowledge, and topic interest were measured. Then, participants were randomly assigned to two conditions, one prompted, in which they were asked planning questions, and one non-prompted. Afterward, participants were assigned six documents presenting conflictual positions on flu vaccination, with the instruction of reading them, writing an argumentative essay, and making trustworthiness evaluations. According to the results, prompting students had a detrimental effect on argumentative essay performance, but not on trustworthiness judgments. This effect was stronger, the higher students' prior beliefs were and the lower their task-value motivation was.