The importance of plan inference in models of conversation has been widely noted in the com-putational-linguistics literature, and its incorporation in question-answering systems has enabled a range of cooperative behaviors. The plan inference process in each of these systems, however, has assumed that the questioner (Q), whose plan is being inferred, and the respondent (R), who is drawing the inference, have identical beliefs about the actions in the domain. I demonstrate that this assumption is too strong and that it often results in failure not only of the plan-inference process, but also of the communicative process that plan inference is meant to support. In particular, it precludes the principled generation of appropriate responses to queries that arise from invalid plans. I present a model of plan inference in conversation that distinguishes between the beliefs of the questioner and the beliefs of the respondent. This model rests on an account of plans as mental phenomena: “having a plan?? is analyzed as having a particular configuration of beliefs and intentions. Judgments that a plan is invalid are associated with particular discrepancies between the beliefs that R ascribes to Q, when R believes that Q has some particular plan, and the beliefs that R herself holds. I define several types of invalidities from which a plan may suffer, relating each to a particular type of belief discrepancy, and show that the types of any invalidities judged to be present in the plan underlying a query can affect the context of a cooperative response. The plan inference model has been implemented in SPIRIT, a System for Plan Inference that Reasons about Invalidities Too, which reasons about plans underlying queries in the domain of computer mail.