Protagoras is a strangely disjointed text. On a first reading, the different sections of the dialogue may seem to have little to do with each other. In fact, connections do exist between these apparently disparate parts, although they tend not to be on the level of narrative, explicit argumentative theme, or literary style. The dialogue begins with a conversation between Socrates and an anonymous friend. The rest of the dialogue is presented as being told by Socrates to this friend. However, the friend does not reappear at the end of the dialogue, so that the text is oddly unfinished as a narrative. The story Socrates tells involves the presence in Athens of the famous Sophist Protagoras, at the time the most famous thinker in Greece. Socrates relates how he is awoken by a friend, Hippocrates, who is excited by the arrival of Protagoras, and who intends to become Protagoras's disciple. But when Socrates questions Hippocrates as to what he hopes to learn from Protagoras, Hippocrates is unable to answer. The two set out to ask Protagoras himself exactly what it is he teaches.

The main dialogue begins when Socrates starts to question Protagoras about what he teaches his pupils. Protagoras asserts that he educates his students in politics and in how to manage personal affairs. But Socrates questions whether this is really a subject that can be taught. Protagoras responds by giving a long speech about the creation of the world. Virtue is indeed teachable, argues Protagoras, because political systems are founded on the basis that all citizens can possess virtue. Similarly, systems of criminal justice are based on the notion that people can be reformed—that is, taught how to be virtuous. Socrates then shifts the subject of discussion to focus more precisely on what virtue is exactly. Is it one thing, or many things? But this line of argument has not progressed very far before the dialogue breaks down completely. Socrates and Protagoras begin bickering over how long their answers to each other's questions should be: Socrates favors short answers and rapid questioning, while Protagoras prefers to be able to answer at length. Other speakers interpose, and persuade the two to return to the subject at hand. Protagoras will question Socrates first, and then Socrates will take his turn.

Protagoras takes this opportunity to switch the subject to a poem by Simonides. Having pointed out a contradiction, Protagoras challenges Socrates to respond. Socrates's argument is ingenious: he interprets the poem as reacting to the assertion of Pittacus, a sage, that it is hard to be good. In Socrates's interpretation, the poem contends that it is hard to become good but that it is impossible to be good all the time, for humans are forced to behave badly by inevitable misfortunes. Socrates then elaborates: misfortune here does not mean poverty or scarcity, but ignorance. As Socrates argues, the only evil is a lack of knowledge, because it is impossible to behave badly knowing what is good.

Socrates continues to pursue this line of reasoning when it is his turn to ask Protagoras questions. The only good, Socrates reasons, is pleasure; to commit an evil action is to unwittingly opt for pain over pleasure. What is needed is a technique that can unfailingly identify what is the more pleasurable course of action in any situation. Having gained Protagoras's assent to this position, Socrates then argues that cowardice is failing to fear the right things, and fearing things that should not be feared. Courage, too, is therefore a form of knowledge. Protagoras has previously accepted that wisdom, temperance, justice and holiness all name the same thing: virtue. Socrates has proved (at least to the satisfaction of Protagoras) that courage is also synonymous with these other terms, and that virtue itself is simply another name for knowledge. If virtue is knowledge, then it can be taught. Thus, both Protagoras and Socrates end up arguing the opposite of their positions at the beginning of the text, and the dialogue ends with Socrates complaining about a missed appointment.