The Story Stops Mid-Sentence
Zeus calls the gods together to decide how to punish Atlantis, and just as he starts to speak, the text ends.
The moment before Atlantis is judged; the dialogue was written about 360 BCE
What the text says
Zeus, the god of gods, who rules according to law, and is able to see into such things, perceiving that an honourable race was in a woeful plight, and wanting to inflict punishment on them, that they might be chastened and improve, collected all the gods into their most holy habitation
Critias 121b-c
And when he had called them together, he spake as follows
Critias 121c
and has the very great advantage of being a fact and not a fiction?
Timaeus 26e
The story
Back on the island, before the war and the sinking, the gods are watching. Zeus, king of the gods, sees that the once noble kings of Atlantis have gone bad. He wants to punish them, but in a way that might teach them to be better.
He calls all the gods to his home at the centre of the universe, from where everything can be seen. They gather. Zeus begins to speak.
And there the story ends. We never hear what Zeus said, how the war was fought in detail, or how the gods chose the earthquake and the flood. The Critias stops in the middle of a sentence.
Earlier, in the Timaeus, Socrates had praised the tale for being 'a fact and not a fiction'. Plato does not say why he never finished it.
Nobody knows for sure why the Critias is unfinished. Plutarch wrote that Plato started the Atlantis work late and died before he finished it. Ever since, people have tried to find Atlantis; one of the most famous was Ignatius Donnelly, whose 1882 book argued it was real, an idea that modern scholars reject. Jowett, who translated these texts, thought Atlantis existed only 'in the imagination of Plato'.