You can read Plutarch's On the Genius of Socrates in English at the Perseus Project.
The dialogue opens with Archidamus asking Casiphias to describe to him the entire plan by which the liberation of Thebes took place, from beginning to end. Casiphias agrees, but waits to launch into his tale until he has been introduced to the other people with Archidamus: Lysithides, Timotheus, the sons of Archinus, and some unnamed friends. Then Casiphias asks how much they already know about the topic.
Archidamus replies that they know how the Spartans surprised the Thebans in a military maneuver and took over the city, ruling it in ways contrary to justice, and that they know what they have gathered from those who were exiled. So they only need to know what Casiphias and his associates actually did to make it possible for the exiles to return and to seize control of the government again.
Casiphias replies that at that time all the plotters met in secret at the house of Simmias; they met there because he had a knee injury that made it difficult for him to move about. Their cover story was that they were meeting for philosophical discussion; to make the cover story stronger, they even occasionally invited some of the Spartans who liked philosophical conversation. Through a system of messages through Charon, the brother of the important Theban general Epaminondas, they discovered that the exiles were set to return. Theocritus the soothsayer remarks to Casiphias that it is surprising that Charon, who is not philosophical by nature, nonetheless is willing to go to such risks for his city, while Epaminondas seems to do nothing; but Casiphias replies that this is simply a matter of differences in reasoning:
Courageous Theocritus, we do what upon mature deliberation we have approved, but Epaminondas, being of a contrary opinion and thinking it better not to take this course, rationally complies with his judgment, whilst he refuseth to meddle in those matters which his reason upon our desire cannot approve, and to which his nature is averse. Nor can I think it prudent to force a physician to use fire and a lancet, that promiseth to cure the disease without them.
Epaminondas has not signed on with the plotters because their plan is going to require deliberate killing of Thebans without trial. Because of the importance of Theban freedom, he does not interfere with their plotting, but he himself refuses to participate in it, preferring to look for an opportunity to rid Thebes of the Spartans and their puppets in a more just way. Anaxidorus hushes Theocritus and Casiphias, noting that Archias and Lysanoridas the Spartan are coming this way. Archias calls Theocritus aside and talks with him and Lysanoridas in private for a long time -- for so long a time, that the conspirators begin to fear that their scheme has been discovered. This gives Phyllidas, however, who is secretary to Archias but also one of the plotters, the chance to get caught up on the situation with the exiles from Casiphias. He himself has planned a feast for Archias to make sure that Archias will be drunk when the exiles surprise the city, but he hasn't been able to drawn Leontidas in as well, so it will be necessary to divide their forces into two groups in order to get both Archias and Leontidas at once. Casiphias agrees to pass this on, but wonders what Archias is talking to Theocritus about. Phyllidas does not know, but he does know that the Spartans, who are famous for paying attention to omens and oracles, have reportedly been worried because of omens and oracles indicating that disaster would come on Sparta.
At this point, Theocritus returns and Phidolaus the Haliartian comes in as well, saying that Simmias asks them to wait a moment, because he is currently pleading with Leontidas to lighten the capital punishment for Amphitheus to banishment. This leads to a curious turn in the conversation; Theocritus remarks that Phidolaus must have been present at the opening of the tomb of Alcmena when the Spartans took its relics back to Sparta and asked what he saw. Phidolaus replies that he was not there, refusing to be a party to the impiety, but there was no body in the tomb. There was, however, a bronze tablet with ancient writing on it, and the Spartan king Agesilaus had sent to Egypt to try to discover what it said. Not long afterward there was a great famine and the area flooded, which the Haliartii believed was a judgment on Sparta for their impiety. Theocritus responds that this was what he was talking to Archias and Lysanoridas about; omens are suggesting that Sparta is under judgment, and thus Lysanoridas will be going to Haliartus to perform rites to Alcmena and Aleus, and then to return and find the tomb of Dirce, but Theocritus thinks he will not find it.
At this point Leontidas leaves, and they go in to see Simmias. Simmias is troubled because his petition has been denied. He then asks if Casiphias knows who the stranger is. Casiphias has no idea what he is talking about, so Simmias replies that Leontidas had said that a man was seen coming out of the tomb of Lysis with a long train of attendants, and that when they went to investigate they found traces of burnt sacrifices. Nobody knows who it is. Phidolaus asks Simmias what he knows about the bronze plaque found in the tomb of Alcmena. Simmias replies that he doesn't know anything about the tablet itself, but he does know that there has been back-and-forth between Sparta and Egypt, and that the language is from the days of Proteus and Hercules. The message on the tablet said something about instituting games in honor to the Muses, and the interpretation that was given was that the Greeks ought "to live peaceably and at quiet, to contend in philosophy to the honor of the Muses, and, laying aside their arms, to determine what is right and just by reason and discourse." That this seemed to be right was confirmed by the fact that it solved an otherwise puzzling oracle from Apollo: "Then the Delians and all the other Greeks should enjoy some respite from their present evils, when they had doubled the altar at Delos." This seemed to be an admonition that the Greeks should study geometry rather than fight. Casiphias's father Polymnias comes in with a message from Epaminondas, asking them to provide hospitality for an important Pythagorean from Sicily who came to Thebes to make offerings at the tomb of Lysis.
At this point Galaxidorus (I'm beginning to think that everyone in Thebes happens to be congregating at Simmias's house, the cast of characters is getting so large) cries out that it is apparently impossible to find a man without superstition. All these omens and oracles and sacrifices only exist to give people in power a way to control the vulgar and the stupid, but astonishingly there are people trained in philosophy who get taken in by them, so that philosophy, which starts out promising to investigate everything by reason, eventually falls back into unreason and dreams and appeal to the gods. Thus, he says to Simmias, "your Socrates" had the right idea. Theocritus protests that this is just the accusation Meletus made against Socrates, that he despised all divine things. Galaxidorus responds that it's not that Socrates despised divine things, but receiving the dream-filled Pythagorean philosophy, he applied the standard of reason to it. Theocritus replies that this doesn't make any sense of Socrates' daimonion, and he gives some stories about Socrates following the counsel of his divine sign.
Galaxidorus dismisses this. The 'divine sign' was just Socrates' experience combined with reason:
For as one grain doth not incline the balance by itself, yet added to one of two weights that are of equal poise, makes the whole incline to that part; thus an omen or the like sign may of itself be too light to draw a grave and settled resolution to any action, yet when two equal reasons draw on either side, if that is added to one, the doubt together with the equality is taken off, so that a motion and inclination to that side is presently produced.
Polymnias remarks that Galaxidorus heard a story from a Megarian, who got it from Terpsion, that Socrates' daimonion was actually just sneezing -- if someone sneezed to the left, he took that as a sign that he should stop doing what he was doing. But this is certainly an odd story -- if that's all there was, why not just say so rather than make all this fuss about a divine something-or-other? Nor does it seem plausible that a wise man would be so moved by something as insignificant as a sneeze. Galaxidorus dismisses the skepticism of the others, however, responding that little things can be quite significant. What is actually happening is that, since we don't really know the sources of our guesses, we attribute their success to divine power; the sneeze was just a sign, and it's not surprising that Socrates attributed it, as the source of his very successful guesses, to a divine power. But he's willing to hear what Simmias would say about it.
However, they are interrupted by Epaminondas, who is coming with friends and the stranger from the tomb of Lysis, whose name turns out to be Theanor.
(to be continued)