Plato's Sophist is explicitly framed as a continuation of his Theaetetus—occurring on the next day and continuing the previous discussion. The two dialogues form a trilogy with Statesman and address related philosophical problems. The two works are considered among Plato's "late" dialogues and are assumed by many scholars to have been written towards the end of Plato's career. They represent Plato's developed and mature thought and are often assumed to have progressed beyond the initial Socratic mode of inquiry (present in the early dialogues) and the "theory of forms" of the middle dialogues to a more complex analysis of language, ontology, and epistemology.

Unlike in the earlier dialogues of Plato, the participants do not include subjects of satire but are instead all admirable characters cooperatively seeking knowledge. The main participants in the dialogue are as follows

There is the Eleatic Stranger, who is a nameless visitor from Elea—home of the philosophers Parmenides and Zeno. He is the major speaker of the dialogue and represents the positions of the Eleatic school of philosophy, especially its belief that knowledge can only be of unchanging being and not of changing phenomena or things that are becoming. The dialogue portrays Parmenides and the stranger quite favorably.

Socrates is also a major participant. Unlike the early dialogues, however, Socrates here appears as a relatively young man learning from the Eleatics, rather than as a central figure of the dialogue.

A minor character is Theodorus; he is an elderly and distinguished mathematician.

Another minor character is Theaetetus. This brilliant young mathematician—who is represented as dying from war wounds in the frame of Theaetetus—is portrayed as a sort of ideal student: modest, intelligent, and energetically pursuing knowledge.

The explicit narrative arc of the dialogue is to seek out a definition of the "sophist," a term that refers to professional teachers of rhetoric. Plato contrasts sophists—who deal in opinion and offer knowledge in exchange for money—with philosophers, who seek unchanging truth and are not financially motivated.

The bulk of the dialogue, though, is concerned with the ontological and epistemological foundations of the two subjects. The opinions of the sophists are held to mingle being with non-being and thus cannot be true knowledge, while the philosopher is concerned with being, or timeless truths. Language is treated as problematic in the dialogue; one of the major thrusts of the argument is that, in order to do philosophy effectively, it is necessary to reflect carefully on language and use it more precisely than one does in ordinary speech.