Many translations of this dialogue are available in the public domain. See, for instance, Project Perseus' copy.
MENO: Can you tell me, Socrates, can virtue be taught? Or is it not teachable but the result of practice, or is it neither of these, but men possess it by nature or in some other way? (70a, Grube trans.)
Meno, as a socratic dialogue, is driven by this question. The character Meno is a student of the sophist Gorgias, and undoubtedly thought he learned the nature of virtue from him. Of course, as Socrates is quick to point out, there is no balm in Athens. Asking Socrates a straight question and expecting an answer is like asking the moon to be full forever. Characteristically, Socrates claims ignorance of whether virtue can or cannot be taught, and so begins the dialogue.
What is virtue?
First, however, Socrates sends Meno on a wild goose chase, looking for the nature of virtue. Typically it doesn't take quite as long for a interrogator to admit their own ignorance of the subject at hand, but Meno is more stubborn than the typical interrogator. He puts forth several theories of the nature of virtue, all of which Socrates slices to shreds. Meno's main error is defining virtue circularly, so that virtue is "acting virtuously" or "acquiring things with virtue" — clearly no definition at all!
The first section ends with Meno lamenting his state of perplexity and the aporia that always follows socratic dialogues. Socrates accordingly professes his own ignorance. Meno then poses the following dilemma:
MENO: How will you look for it, Socrates, when you do not know at all what it is? How will you aim to search for something you do not know at all? If you should meet with it, how will you know that this is the thing that you did not know? (80d)
Plato (through the mouthpiece of Socrates) then presents the doctrine of the immortality of the soul as a premise into the dialogue. Later, when Plato wrote The Republic, he takes the doctrine of recollection (where the Meno eventually leads) as evidence of the immortality of the soul, which does seem to make the argument a bit circular. At most, if you assume either doctrine, we can say the other follows.
The doctrine of recollection says that what we call learning is actually just remembering. Our immortal soul knows essentially everything, but in being tied to a body it forgets most of it. So teachers have to set out to cause their students' souls to re-remember part of the knowledge that the soul has forgotten. Therefore, it is possible to learn what virtue is because we've always known what virtue is, it's just that we've forgotten it for the moment.
Double the square, slave¹
To illustrate this doctrine, Socrates teaches one of Meno's slave boys a basic theory — how to double the area of a given square² — of geometry only by asking questions. This is interesting, because the end result of socratic dialogues is supposed to be aporia, (that is, the acknowledgement of ignorance) yet in the slave boy's dialogue, he "recalls" the truth in the end.
As I've mentioned before, the doctrine of recollection is quite important to Plato's later works. But using this anecdote as evidence for the doctrine of recollection seems to fail, because Socrates' character uses complex questions to lead the slave along the path of "recalling" something they never knew in the first place. A typical example of a complex question is: "Have you stopped beating your wife?" Such questions, when answered, lead the interrogated into claiming knowledge of something (in this case, that they know they beat their wife) even if the fact isn't true.
Even with this obvious fallacy, I hesitate to say that Plato intended for the doctrine of recollection to be a joke. For this to work, you'd have to ascribe to Plato a great deal of sarcasm. After all, if recollection is a joke, then so is the immortality of the soul, the Platonic forms, and pretty much the rest of his philosophy. There are a few indications that this could be the case (I refer the reader to jooky's or SciPhi's writeups in The Republic, for examples of silly things Plato asserts) but there is no clear textual evidence one way or another.
A new interrogator, Anytus is introduced into the dialogue. Plato doesn't like him. Socrates uses him to present several instances of virtuous men who fail to teach their loved ones virtue. Anytus tries to defend them, fails horribly, and runs away like a spoiled brat who's had his lunch money stolen. What a moron.
To end the dialogue, Socrates presents the difference between knowledge and correct belief. Some people believe that Socrates may have been a stonemason by profession from this next analogy, repeated in The Republic:
SOCRATES: [You don't understand the difference between knowledge and correct belief] because you have paid no attention to the statues of Daedalus, but perhaps there are none in Thessaly.
MENO: What do you have in mind when you say this?
SOCRATES: That they too run away and escape if one does not tie them down but remain in place when tied down. (97d-e)
According to the myth, Daedalus (whom you may recall built The Labyrinth, had a son named Icarus, and was an Eschelon AI network in Deus Ex) built statues that moved around on their own. Correct belief is like this statue because it is uncertain. On the other hand, when you tie the statue down, it cannot move anymore. That is what knowledge is like, according to Plato — a tying down of correct belief.
So finally, Socrates concludes the dialogue by saying that what he can teach through questioning is only correct belief, and that true knowledge can only come through self-reflection. This is consistent with Plato's belief that language is inadequate for teaching knowledge (writ large in The Republic), but that language can be used to foster correct belief. This, I believe, is why the Meno lacks as much emphasis on aporia as some other Socratic dialogues: it is implicit that it is only belief being taught, not knowledge.
¹ I'm indebted to SciPhi for the title of this section, which he used as a crossword puzzle hint once.
² Supposedly I'm a mathematician on the side, but honestly, this part of the Meno isn't even interesting to me. But, if you really want to know how to double the square, I suppose I'll explain. Take a square, and construct the square whose side is twice that of the given square. Then find the midpoint of each side of the bigger square and connect the four together. The bigger square's area is four times that of the smaller; the inner diamond made by the midpoints is half the bigger square. So the diamond is twice the area of the smaller square.