C. S. Lewis subtitled his novel A Dream. Ostensibly the first-person narrative of a passenger on a fantastic bus trip, the story may be interpreted as an allegory of the spiritual journey to either enlightenment or perdition during one's lifetime; in other words, the journey towards the fate of one's immortal soul.

After the unnamed narrator randomly joins a queue with a diverse and ever-changing array of strangers, they all board a bus. They converse with their seat-mates about the merits of their destination and the other passengers' critical attitude toward it. Suddenly they find that the bus is airborne:

"Hullo! We've left the ground."

It was true. Several hundred feet below us, already half hidden in the rain and mist, the wet roofs of the town appeared, spreading without a break as far as the eye could reach.

The narrator is surprised to realize following a long, rambling, autobiographical monologue by another passenger (who is radical in their politics but is critical of the Soviet system) that this person is actually dead. The storyteller continues his tale—which includes his unsuccessful education, lackluster career, and, finally, failed love—all the way through to the point of his own suicide:

And he had been very badly treated by a girl, too. He had thought her a really civilized and adult personality, and then she had . . . revealed that she was a mass of bourgeois prejudices. . . . She had even shown herself, at the end, to be mean about money. That was the last straw. He had jumped under a train.

I gave a start, but he took no notice.

This storyteller presents the concept of the "grey town," above which they are apparently traveling, which represents the Christian idea of purgatory. He had been sent there because of his suicide but has embarked on this busy trip because of his determination not to end up in the grey town—to advance into heaven rather than to be denied permission to ascend.

The theological arguments for salvation are advanced through the various characters' stories. The narrator ends up in a valley with Bright People who try to enlighten them. They are spiritual guides who have come forth to help relatives and friends reach "the mountains"—the highest spiritual realm, or heaven. They try to team up with the discouraged bus passengers, to whom the narrator assigns various "ghost" nicknames, such as the Bishop Ghost and the Hard-Bitten Ghost.

While the novel can be compared to Dante's Inferno (with Virgil as Dante’s guide), Lewis carefully avoids simplistic parallels and develops an original story in which the narrator's fate is inadequately resolved. As he moves closer to salvation, the landscape and skyscape undergo physical transformations; the lightening sky illuminates the bus interior and alters the narrator's mood:

The greyness outside the windows turned from mud-colour to mother of pearl, then to faintest blue, then to a bright blueness that stung the eyes. We seemed to be floating in a pure vacancy. There were no lands, no sun, no stars in sight: only the radiant abyss.