Hamlet’s father’s spirit reveals that he was murdered by his brother, Claudius, and explains that, because he died unshriven, he must suffer in purgatory until all his earthly sins are purged. Hamlet reacts with shock, incredulity, and hesitancy to the ghost’s demand that he avenge his father’s “foul and most unnatural murder.” The caption quotes from the play, “I am thy father’s spirit. [. . .] Revenge his foul and most unnatural murder.”