This article places Margaret Cavendish's most famous fictional "prose of a certain length," The Blazing World, in the welter of available genres from which she fashioned the fantastic hybrid attached to her 1666 Observations on Experimental Philosophy. In a time when the normative medium for imaginary narrative was verse, she hybridized her text further by writing in prose and attaching it to a work of natural philosophy. This article's chief effort is to reveal the hybridity of the work's satiric pastiche, with its calculated dazzle and challenge to its original readership's "real," and its responsibility in the history of literature in English for the emergence of the novel, not so much and but as science fiction. The work takes a major step toward the constitutive conflict of the modern novel between "inside" and "outside" experience, appearance, action. Its fantasia of the protagonist's cohabitation with her friend inside the body of her husband delights with a gender fluidity or boundary breakdown like that of its own form, followed by a spectacle of sublime female military power and violence on another world.