The Theaetetus and the Sophist both stand in the shadow of the Parmenides, to which they refer. I propose to interpret these two dialogues as Plato's first move in the project of reshaping his metaphysics with the double aim of avoiding problems raised in the Parmenides and applying his general theory to the philosophy of nature. The classical doctrine of Forms is subject to revision, but Plato's fundamental metaphysics is preserved in the Philebus as well as in the Timaeus. The most important change is the explicit enlargement of the notion of Being to include the nature of things that change.This reshaping of the metaphysics is prepared in the Theaetetus and Sophist by an analysis of sensory phenomena in the former and, in the latter, a new account of Forms as a network of mutual connections and exclusions. The division of labor between the two dialogues is symbolized by the role of Heraclitus in the former and that of Parmenides in the latter. Theaetetus asks for a discussion of Parmenides as well, but Socrates will not undertake it. For that we need the visitor from Elea. Hence the Theaetetus deals with becoming and flux but not with being; that topic is reserved for Eleatic treatment in the Sophist. But the problems of falsity and Not-Being, formulated in the first dialogue, cannot be resolved without the considerations of truth and Being, reserved for the later dialogue. That is why there must be a sequel to the Theaetetus.