You can read Hipparchus online in English at the Perseus Project and in French at Wikisource.
Solon gave Athens law, and then left for a journey that lasted ten years. During that time, Athens descended into confusion as faction fought faction. Pisistratus, a war hero and one of Solon's relatives, backed by the poor of the city, managed to impose order and become tyrant, sole ruler of Athens. One of his major accomplishments was exiling the Alcmaeonids. As it happens, he was quite effective: matters of government were run well, taxes were cut, the class tensions in Athens were diffused. He was a patron of the arts, and under his government they thrived -- dithyramb, tragedy, and more. He also had Homer copied and archived. He was fairly popular. When he died and his sons, Hippias and Hipparchus took over, they were not so popular, and the Alcmaeonids were plotting to return. The exact details of what happened are not clear, and our main sources (Herodotus' Histories, Thucydides' History of the Peloponnesian War and Aristotle's Athenian Constitution) disagree on a number of important details; Thucydides himself makes clear that the Athenians actually had several different versions of the story floating around, since he uses the story at the beginning of his work as an example of the historical ignorance of most Athenians. But the common story seems to be that Hipparchus had made advances on a young man named Harmodius and been rebuffed. In retaliation, Hipparchus chose Harmodius's sister to carry the basket in the Panathenaeia (a great honor), but when it came time to do it, he refused to allow it, claiming in public that she was not, as the ceremony required, a virgin, and then chasing her away. In response, Harmodius and his lover/mentor (erastes) Aristogeiton decided to kill Hippias and Hipparchus. As it happens, they only managed to kill Hipparchus. Hippias initiated a very strict regime in response, which decreases his popularity. In the meantime, there was a fire at Delphi, and the wealthy Alcmaeonids paid for rebuilding; this gave them an opportunity to suborn the Oracle at Delphi. They bribed the Oracle into telling every Spartan who came to the Oracle that Athens must be freed from Hippias. Sparta was an ally of Hippias's government; but even the Spartans were not cautious and stubborn enough to ignore the repeated insistence of the god Apollo. So they invaded and Hippias was forced to flee. The Alcmaeonids came back and instituted the democratic regime (they probably wanted an aristocracy, but they had to compromise). In Athens, therefore, the Tyrannicides, Harmodius and Aristogeiton, came to be celebrated as heroes who had liberated Athens and made the democratic regime possible, in part due to the influence of the Alcmaeonids. Pericles, of course, was a member of the Alcmaeonid family, as was Alcibiades.
(in order of appearance)
Socrates converses with an anonymous interlocutor.
The Plot and The Thought
Socrates opens the dialogue asking what greed is, and who the greedy people are. His interlocutor responds that they are people who treat as gain things of no value. This is puzzling, Socrates points out, because if they treat valueless things as gains without knowing that they are valueless, then it seems greedy people are stupid; but if they treat valueless things as gains while knowing that they have no value, there doesn't seem to be anyone like that. To this the interlocutor replies that the greedy are those with an insatiable desire to gain, regardless of the value of things. But this just raises the same point, so greed must be a kind of ignorance.
But the puzzle gets more complicated. Greedy people love gain, which is the opposite of loss, which harms people; and to get a good is a gain, so the greedy people are the people who love what's good. But if that's the case, it seems that everyone's like that. The interlocutor protests that good people don't want gains from which they suffer harm. But this simply creates another puzzle: how can it be a gain if it harms you? Socrates accuses his interlocutor of trying to deceive him by saying the opposite of what he means; the interlocutor replies that it's Socrates who is deceiving him.
Socrates says that it would be wrong of him not to obey a wise man. This brings us to the tale of Hipparchus. According to Socrates, Pisistratus's son, Hipparchus, was the oldest and wisest of his chidren. He was the first to bring in Homer's works, and he made the rhapsodes recite them at the Panathenaea; he enticed Anacreon and Simonides to Athens. In addition, he set up Herms (statues of Hermes, god of travel) along the sides of the road, and put his own wisdom on them:
He did this in order that, first, his citizens would not be impressed by those wise Delphic inscriptions, "Know Thyself," and "Nothing in Excess," and other things of this sort, but would instead regard the words of Hipparchus as wiser. And, second, he did this so that when they travelled back and forth they would read and acquire a taste for his wisdom and would come from the country to complete their education. There are two sides to the inscriptions: on the left side of each Herm, it is inscribed that the Herm stands in the middle of the city or the deme, whereas on the right it says: "This is a monument of Hipparchus: walk with justice in mind." There are many other fine inscriptions of his poetry on other Herms. There is one in particular--on the Stiria road--on which it says: "This is a monument of Hipparchus: do not deceive a friend." So, since I am your friend, I would never dare to deceive you and disobey such a great man. (228d-230b)
After the death of Hipparchus, Hippias ruled for three years, which were in the ancient days the only years of tyranny, and "during the other times the Athenians lived almost as when Cronus was King" (229b). Socrates denies the basket story, saying that more sophisticated people claim that Harmodius had become the favorite of Aristogeiton, and the later prided himself on educating Harmodius, regarding Hipparchus as a rival. Harmodius in the meantime had become lover of someone-or-other; Socrates can't remember his name. The youth had thought Harmodius and Aristogeiton wise, but when he began associating with Hipparchus, he was no longer impressed by them; so Harmodius and Aristogeiton killed Hipparchus.
The interlocutor replies that Socrates either does not regard him as friend, or is disobeying Hipparchus, since Socrates will never persuade him that he is not deceiving him with these arguments. Socrates says he'll let the interlocutor take back anything he wants from the previous discussion. Socrates goes through a list of possible things the interlocutor could take back, and finally suggests that instead of taking gain to be good, they say that some gain is good and some bad. Since they are both, by supposition, genuinely gain, we have to determine what makes them both so. Socrates suggests that it might be either getting things without spending anything or receiving more when one spends less. But again, not just anything you could get would count as a gain; for instance, if you 'gain' sicknesss at a feast for which you do not pay. So it seems we are back to the claim that only good things can be gains. Likewise, it's not just 'more' that is important: to give gold and get double the weight in silver is not a gain if gold is worth by weight twelve times what silver is. It needs be valuable. But if the valuable is what gain is, then we need to know what's valuable to possess. But it seems that what is valuable to possess is whatever is beneficial. So people are greedy when they want what is beneficial; which all virtuous people do. And so it seems that both the wicked and the virtuous are greedy.
* Thucydides explicitly denies that Hipparchus was the oldest of Pisistratus's children; Hippias was. This fits what we know from other sources. Likewise, Hipparchus never actually ruled; only Hippias did. Likewise, Pisistratus was tyrant before Hippias. But the claim that the Athenians, outside of Hippias's 'three years', lived like they did in the Golden Age is so absurdly exaggerated, and the over-the-top terms in which Socrates describes Hipparchus's 'wisdom' are clearly not intended to be taken seriously. And while it's not wholly surprising that Socrates reads the erastes/eromenos relations primarily in their educational sense, the more 'sophisticated' story about why Hipparchus died is so obviously implausible it has to be deliberate. (It's worth noting that the very brief allusions to it in Plato's Symposium are also inconsistent with it.) So we seem to have a case of Socrates deliberately telling his friend a lying story with the moral 'do not deceive a friend'.
* The road to Stiria is a nice touch; apparently it went out to a stone quarry, which Socrates as stonecutter would no doubt have known.
* Aristophanes mentions in The Wasps that Simonides received payment from Hipparchus for his poetry, and regards it as a scandal; so Hipparchus's patronage of the arts also fits with the theme of greed.
* In Thucydides the Hipparchus story is closely linked with the Syracusan expedition -- and Alcibiades, of course, was accused of mutilating the Herms on the eve of that expedition. It is unlikely that the same association would not have occurred to the author, so perhaps part of the point of the dialogue is to link the Syracusan expedition to philokerdia? That's remarkably subtle, if so.
* I think it's worth remembering Gorgias, and Socrates's insistence there that what people like (or prefer) is not what they want; it seems to be closely related to the issues related by this argument. On the account there, everyone wants what is genuinely good, so unjust people necessarily are not getting what they want, even if they get what they like to have.
Quotations are from Nicholas D. Smith's translation in Plato, Complete Works, Cooper & Hutchinson, eds., pp. 609-617.