Thomas enters a dream state where he finds himself in complete blackness devoid of time or physical sensation. After experiencing this void for an unknowable amount of time, he feels a wind pick up and swirl around him, ripping at his clothes. A tower of mist moves towards him and he gets absorbed by it. At that moment, all his memories come flooding back along with an immense amount of pain.
Since nothing exists in the Changing except the self, the Changing represents moments of intense introspection in which people discover their true selves. For most Gladers, introspection makes them sullen and hopeless. But, if Thomas wants to grow up and save the Glade, he must learn how to be introspective, uncover the truth of his identity, and not give up hope.