You can read Protagoras online in English at the Perseus Project and in French at Wikisource.
There is a frame narrative in which an unnamed interlocutor talks with Socrates, after which Socrates narrates the events of the gathering of Sophists. The dialogue depicts a fairly large gathering, so there are over twenty named characters. They can easily be grouped, however. Notably, everyone who will be in the Symposium, except Aristophanes, is in attendance; this may be due, however, less to a connection with that dialogue than to the fact that both dialogues have a number of characters associated with the desecrations of 415 that led to many wealthy young men fleeing Athens and Alcibiades defecting to Sparta.
Hippocrates, son of Apollodorus
A wealthy young Athenian. Debra Nails suggests that he might be a nephew of Pericles -- there are quite a few members of Pericles' family at the gathering, and 'Hippocrates' occurs several times in the known family tree. It would also explain a few things in the text.
This is the same Critias from Charmides, Plato's relative, who would later become one of the Thirty Tyrants.
Alcibiades is very young here, and the dialogue seems to predate any relationship between Alcibiades and Socrates.
This is the same Callias who is Hermogenes' rich half-brother. He is the host of the gathering.
Those with Protagoras
Protagoras of Abdera
While Protagoras was born far to the north, he seems to have lived a considerable part of his life in Athens. He claimed to teach rhetoric and virtue. He seems to be around sixty years of age at the time of the dialogue.
Xanthippus and Paralus, sons of Pericles
The sons of Pericles' first wife. Both of them would die within a few years in the Great Plague at the beginning of the Second Peloponnesian War, which makes Protagoras's comments about their age perhaps a bit poignant.
Plato's uncle, the same Charmides after whom Charmides is named. He became one of the Ten Tyrants ruling the Piraeus under the Thirty Tyrants of Athens.
Philippides of Paeania and Antimoerus of Mende
Philippides is known to be from a wealthy family, but nothing else is known of the two.
Those with Hippias
Hippias of Elis
Hippias the polymath is the same sophist who is mocked mercilessly in Hippias Major and Hippias Minor.
Eryximachus of Athens
The same as the character in the Symposium; he was a physician and a friend of Phaedrus. He would be implicated in the sacrilege of 415 and have to flee Athens.
Phaedrus of Myrrhinus
The same Phaedrus from Phaedrus and the Symposium. He would be implicated in the sacrilege of 415 and have to flee Athens.
Andron of Gargettus
He is the Andron mentioned in passing as an associate of Callicles in Gorgias. Most of what is known about him is really about his son Androtion, who was a wealthy and famous writer. Andron himself, however, seems to have been put in prison for debt at some point. He would become a member of the Four Hundred who overthrew the democratic government of Athens in 411.
Those with Prodicus
Prodicus of Ceos
Prodicus is the sophist who is consistently treated most favorably in both Plato and Xenophon. He was famous for always making distinctions.
Pausanias of Cerameis
The same Pausanias as in the Symposium. He is the lover of Agathon.
Agathon of Athens
The same Agathon whose victory is celebrated in the Symposium.
Adeimantus son of Cepis and Adeimantus son of Leucolophides
Nothing much is known about these two, but Adeimantus son of Leucolophides was the Adeimantus who was implicated with Alcibiades in the mutilation of the herms in 415. His property was confiscated, and by chance the record of it has survived, showing that he was a very wealthy person with several slaves.
In addition, there are a number of other unnamed participants and at least one unnamed (and grouchy) slave.
The Plot and The Thought
An anonymous interlocutor remarks to Socrates that he is sure Socrates has been hunting Alcibiades, and asks if the youth favors Socrates in any way. Socrates remarks that Alcibiades had come to his support in a discussion earlier today, but at the time he wasn't paying attention to Alcibiades, but to someone far more beautiful, because more wise -- Protagoras. Protagoras has been in town for two days, and Socrates has had a long conversation with him earlier that day. So they sit down and Socrates tells the story.
It was before dawn and Socrates was still in bed when Hippocrates came into his bedroom, full of excitement, because he had discovered that Protagoras was in town. The reason Hippocrates has come to Socrates is that he wants to meet Protagoras, but he's very young, and so he's hoping that Socrates will introduce him, since Protagoras is staying at the house of Callias. Socrates remarks that it's a bit too early, so they walk around the courtyard until after dawn. While they do so, Socrates presses Hippocrates about what he wants to become, noting that if Hippocrates went to Hippocrates of Cos (of Hippocratic Oath fame) to learn, he would be made a physician. Reluctantly, Hippocrates admits that Protagoras is a Sophist and thus that one would apparently learn from him how to be a Sophist, which would be a shameful thing to be; but Socrates suggests that perhaps what he is looking for is a more general education suitable to a gentleman's life. When Hippocrates agrees, however, Socrates makes clear in another line of questioning that this is even more serious -- he's about to hand his soul over to a Sophist, even though he can't even clearly identify what a Sophist would do with it. He ends by noting that he and Hippocrates are a bit too young to handle the problem without the help of their elders, and so they set out for Callias's house.
When they get there, the slave almost refuses to let them in, grumbling about all the Sophists who have been coming. Inside, they see quite a comical picture. Protagoras is walking in the portico with a crowd of people following him:
He enchants them with his voice like Orpheus, and they follow the sound of his voice in a trance. There were some locals also in this chorus, whose dance simply delighted me when I saw how beautifully they took care never to get in Protagoras' way. When he turned around with his flanking groups, the audience to the rear would split into two in a very orderly way and then circle around to either side and form up again behind him. It was quite lovely. (315b)
On the other side of the colonnade is Hippias of Elis, sitting in a high seat and taking questions on astronomy and physics from a group of people around him. Prodicus of Ceos turns out to be still in bed in a side storage room, talking to students seated around him.
Socrates introduces Hippocrates to Protagoras, saying that he wants to be a respected man and is hoping to get this by association with Protagoras; he asks Protagoras if he wants to talk about it privately or not. Protagoras thanks him for his discretion, noting that there is considerable hostility to Sophists, but he also says that unlike other Sophists he himself is quite open about being one. If Socrates and Hippocrates have a request, he would be happy to do it right there. Socrates, of course, is shrewd enough to see that what Protagoras really wants to do is show off in front of Hippias and Prodicus.
The pull together a makeshift auditorium around where Hippias had been teaching, and Protagoras asks Socrates for his question again; Socrates replies that his friend Hippocrates is interested in studying with him, and wants to know what the result of that will be. When Protagoras replies that Hippocrates will get better and better every day, Socrates says drily that this makes sense, but wants to know what Hippocrates will be better and better at. Protagoras replies that Hippocrates will be better at craft of politics (politike techne). Socrates expresses the doubt that this is teachable (although given how he argues as the dialogue continues, this may be ironic), giving the example of the children of Pericles. He asks for a demonstration of this idea, and Protagoras gives the audience a choice of getting a story (mythos) or argument (logos); and when they tell him to do whichever he pleases, he gives them a story.
Protagoras' Great Speech
Originally there were only gods. When they decided to make mortal creatures, they did so by mixing earth and fire and then giving them into the care of two brothers, Epimetheus and Prometheus, so that the latter would give them their appropriate capabilities. Epimetheus begged Prometheus to let him do it himself, and Prometheus agreed. Epimetheus gave some speed, some flight, some strength, and so forth. He did this by compensation (epanison), giving to each what it needed to survive both other creatures and the weather. When Epimetheus got to human beings, however, he discovered that he had run out of gifts. At this point Prometheus arrived to check his work. Discovering human beings left with no natural gifts at all, knowing that the deadline was coming up quickly, Prometheus cheated by stealing the conjoined gifts of craft-wisdom and fire from the workshop of Hephaestus and Athena and giving it to human beings. This craft-wisdom allowed men to do everyday things; but it was not political wisdom, which they lacked entirely. And the old story goes, of course, that Prometheus was put on trial for his theft.
A side effect of human beings having a divine gift is that they, because of their kinship with the gods, were the only animals who worshipped gods and invented things like languages and beds. Originally they all dwelt apart, but the wild beasts were picking them off, so they banded together into cities for protection. But, of course, they had no political wisdom, so the result was that instead of being harmed by wild animals they were harmed by each other. Zeus, afraid that human beings would perish entirely, sent Hermes to earth in order to teach them right (dike) and reverence (aidos). Hermes asked Zeus how he was supposed to distribute it -- should it be given to a few, who then distribute it to the rest, or to everyone. Zeus replied that it should be given to everyone, because cities cannot survive if only a few people have right and reverence; and, further, Zeus decreed that anyone who lacked right and reverence should be put to death. Thus it is that when people want advice in medicine or the like, they look for the few who have it, but if they want advice in how to govern, they allow advice from everyone. And another sign of this is that we regard people without technical skills as merely ignorant; people without political skills we regard as crazy.
However, even though everyone has some of it, it does not arise naturally or by chance but only by training and education. A sign of this is that we take people who lack a good by nature or by fortune to be only unfortunate, whereas we actively blame and punish people who lack justice, the punishment being to deter others from doing the same.
This leaves only Socrates' question about how it is that great men cannot teach virtue to their children, and Protagoras says that he will respond to this by argument rather than story. Essentially, Protagoras argues that they do, in fact, teach their children -- it would be odd, after all if, they taught their children everything but the things that come with a death penalty. The course of education that they use is pretty much what you would expect: they constantly try to teach them the meaning of ethics-words, and if the children get things wrong, they get spanked. Parents then send their children to school to learn poetry, and also music and athletics. Then after school people are trained by the laws of the city.
This shows that everyone does, in fact, take virtue to be teachable. Why, then, do good people often fail? Precisely because of what was said before: everyone is the teacher of justice. In a city in which everyone taught everyone flute-playing, the people who would end up being the best flute-players would just be the ones that had the most natural aptitude, regardless of who their parents were. But everyone in the city would be an at least competent flute player. So it is with justice: everyone being the teacher of justice, the people who end up most just are merely those who have the most natural aptitude for it. But everyone is more just than they otherwise would be. It's just like the Greek language, of which everyone is the teacher. But, says Protagoras, sometimes people are just a little bit better at the teaching, and we should be grateful to them. He counts himself as one of those, and notes that he leaves it to his students to agree -- he'll name a price, but if the student doesn't think it was a fair exchange, they'll go to the temple, the student will swear before the gods what he thinks the real value was, and Protagoras will take that.
Socrates says that he was spellbound by this speech, and after it was finished waited for Protagoras to go on. But, of course, Protagoras was finished, and we all know that Socrates has a reply.
* The scene at Callias's seems to show two themes. The first is that Plato is borrowing the conventions of Greek comedy, complete with a Protagorean chorus. The second is that Plato presents it as a descent into the underworld, even quoting Odysseus on his descent into the underworld. There is probably also some significance in the fact that Protagoras is walking, Hippias sitting, and Prodicus lying down in bed, but I'm not wholly sure what it is supposed to be.
* Socrates regularly uses the example of Pericles unable to teach his children throughout the dialogues. The difference here is that Pericles' children are actually sitting in the audience when he uses the example. Socrates' use of the example here seems to be at least half-joking -- as is the example of Clinias and Alcibiades, Alcibiades being in the audience as well.
* It is done with ingenious subtlety, but Protagoras' Great Speech is a very flattering eulogy of Athenian democracy. Only in a democratic city like Athens is every citizen treated like an expert in justice. And the direct implication of his speech is that, because Athens has education and law-courts, the most unjust Athenian who has ever lived is more just than a barbarian who lacks such accoutrements of Greek culture. Justice is literally like the Greek language (note that teaching the meaning of words is where Protagoras says we begin in teaching children to be just): it is just part of the culture. This is going to play an important role in shaping parts of Socrates' rather mischievous reply, in which (for instance) he treats Sparta as more educated than Athens. It is also why Protagoras and Socrates will go on to argue the interpretation of a poem -- this is not a digression, because poetry is the second stage of the educational scheme for becoming just.
* There was a rumor in antiquity that a significant part of the Republic was taken from Protagoras' book, Truth. We don't know quite what this means since we don't have Protagoras' work, but it's notable that there are similarities between how Protagoras here describes moral education and how Socrates in the Republic does -- and it may well be that in the latter Plato is indirectly criticizing Protagoras' overall view by, so to speak, rewriting it.
* Given that Protagoras is currently in a room filled with a number of his students, it's hard not to read as self-serving his casual remark that we should be grateful for those with better skills for teaching justice, particularly when he goes on immediately to talk about his payment scheme. At the very least, the whole speech, beautiful as it is, comes across in context as something of a sales pitch.