You can read the Epinomis online in English at the Perseus Project and in French at Wikisource.
The dialogue has the same characters as the Laws: the Athenian Stranger, Clinias of Crete, and Megillus of Sparta. Megillus, however, is only present; he does not speak.
The Plot and The Thought
The dialogue opens with Clinias insisting that they should finish the discussion; in particular, they should discuss the most important thing: "what a mortal must learn in order to be wise" (973b). The Athenian replies that most human beings are not happy, and that life tends to be hard and harsh. One of the difficulties it throws in our way is that of discovering wisdom. While some people have been reputed wise by learning various arts and sciences a long time ago, we see in most cases that you can spend lifetimes on the knowledge and not be any wiser. Such is the knowledge relevant to what to eat, or the knowledge involved in agriculture, or the knowledge of architectures and the crafts. Prophetic inspiration does not give it, nor do any of the fine arts, nor does military strategy or navigation. Wisdom is not even given by the natural talent for learning well. But, the Athenian says, there is one kind of knowledge without which human beings would be virtually senseless and unintelligent, namely, the gift of numbers. It is god-given:
It is God himself, I believe, and not some good fortune that saves us by making this gift. But I must say which god I mean, though it will seem strange, though yet in a way not strange....Uranus (i.e., heaven), the god whom above all others it si most just to pray to and to honor, as all the other divinities and gods do. We will unanimously agree that he has been the cause of all other good things fo rus. But we declare that he is really the one who gave us number too, and he will continue to give it, supposing that we are willing to follow him closely. (976e-977a)
Without number we can know nothing of proportion or of how to prove things or of how to give a rational explanation for anything or of how to behave in an orderly and thus virtuous way or of how to make beautiful things. We are taught number directly by the god himself, whether we call him Cosmos or Olympos or Ouranos:
With us humans, the first thing God caused to dwell in us was the capability to understand what we are shown, and then he proceeded to show us, and he still does....Since Heaven never stops making these bodies ply their course night after night and day after day, he never stops teaching humans one and two, until even the slowest person learns well enough to count. (978c-d)
The Athenian then reiterates the account given in Book X of the Laws of the priority of soul, and argues that there are two kinds of living thing: one made of fire and one made of earth. The living things made of fire move in perfect order, whereas the living things made of earth are more disordered. From the fact that the bodies of the heavens move in such excellent order, we should conclude that they are not just living but intelligent. The Athenian notes that the vulgar populace tends to assume that because the stars always do the same thing that they are unintelligent, but replies that this makes no sense if you think about what intelligence is. It is that which is less subject to chance and inexplicable motion, that which is most uniform and invariable, that most deserves the name 'intelligence'. And we can see this confirmed in the fact that the stars are extraordinarily vast, but still keep an orderly and lawlike motion. And if soul is prior to body, as argued in Book X of the Laws, then the bodies that decorate Heaven must move according to souls. There are then two possibilities: either they are themselves gods, or they are likenesses of gods that have been formed by the gods. Thus the stars must be honored and hymned as higher and better and nobler and more beautiful than us. We can legislate about other gods (Zeus, Hera, and so forth) as seems best given our history; but the stars are the gods that are "visible, greatest, most honored, and most sharply seeing everywhere" (984d).
After the living beings of fire come the living beings of ether, then the living beings of air, water, and earth. Between us and the gods are the daemons. Those of ether, who are closest to the gods, and those in the middle position, made of air, are always imperceptible to us. Unlike the gods, they can experience pleasure and pain, love and hate; but they are better at loving good and hating bad than we are. Beneath these two kinds of daemons are the daemons made of water, which are sometimes imperceptible and sometimes perceptible. The gods and the daemons are the sources of all religious rites, and their determinations, such as we can learn of them through dreams or divine voices, should be respected by legislators. This is difficult to do, but the Greeks are well-favored. Because they have education, the oracle of Delphi, and relatively good laws, as well as an excellent location for the viewing of the sky, they will tend to be able to worship the gods better than any other people. This right reverence for the gods is the most important part of virtue.
Astronomy therefore is the science of wisdom, and the true astronomer is the wisest kind of person. Out of astronomy we learn all of mathematics, music, and dance. We learn reverence for the gods, and everything needed to know the bonds that unite the world. After death the astronomer (and here the Athenian says he is half-joking and half-serious) will have the kind of pure knowledge without which human beings cannot be happy. Thus astronomy should be the foundation on which high office and especially the Nocturnal Council in the new colony is based.
* One of the content-based reasons for questioning the authenticity of the dialogue is the extraordinary exaltation of astronomy. Astronomy, to be sure, was clearly stated to be important in the Laws; while the Laws is a work of civil theology, not natural theology, it made clear that astronomy was the point at which civil theology and natural theology overlapped. The two bulwarks mentioned there, erected against the corrosive influence of atheism on our conceptions of rule of law, were the priority of soul over body and the rationality of the heavens. So it makes sense that astronomy would be foundational. But the claims made for astronomy here strike many people as much more extreme than one would expect from Plato, even so. Plato's Socrates, for instance, in the Republic seems to hold (529d-e) that astronomy is merely a preparatory discipline for the much more important knowledge of the Forms. However, the Epinomis is building on comments in the Laws itself, for instance, Book VII (817e-818a).
* The Epinomis is one of the earliest works attesting to our names for the planets -- it uses the Greek names, of course, and we use the Latin, but the correspondences are exact: Aphrodites' star is Venus, Hermes' star is Mercury, Ares' star is Mars, Zeus's star is Jupiter, and Chronos's star is Saturn. (Mercury is also called Hermes' star at Timaeus 38d.)
Quotations are from the translation of Richard D. McKirahan, Jr., in Plato, Complete Works, Cooper and Hutchinson, trs., pp. 1617-1633.
And now we have gone through the entire Platonic corpus.