Warning: Wikipedia contains spoilers
In The Great Divorce, the narrator (Lewis himself) dreams that he is in a grim and gloomy town (a depiction of Hell or purgatory for those leave it). He finds a bus for people who wish to see Heaven, gets on, and converses with some of the other people on the bus. The bus eventually reaches paradise, and the people on the bus realise that they are ghosts, and every feature of the landscape (including drops of water and blades of grass) causes them immense pain.
Shining people come to meet those from the town, and try to persuade them to repent and enter Heaven proper. Most of the ghosts refuse, giving various reasons and excuses. None of the ghosts realise that they have been in Hell.
The narrator is met by George MacDonald. MacDonald explains that it is possible for a soul to remain in Heaven having been in the town; for such souls, their time in the town has been purgatory, and the goodness of Heaven works backwards into their time in the town. Conversely, the evil of Hell works backwards so that if a soul remains in the town, their time on Earth is turned to badness. According to MacDonald, Heaven and Hell cannot coexist in a single soul, and while it is possible to leave Hell and enter Heaven, doing so implies turning away (repentance).