"The Book of Thel" is a narrative poem describing the quest of its protagonist, the virgin Thel, to understand the nature of existence and why beautiful things are fleeting. Thel is the youngest of the daughters of Mne Seraphim and spends much of her time wandering in the valleys, lamenting the fact that everything lasts only a brief time before it dies—"Why fade these children of the spring? born but to smile & fall?"

Thel's lamentations are overheard by the Lily of the Valley, who appears to her and explains that, although she is small and "lowly," she experiences the presence of God in her daily existence and is comforted by it. Thel, she says, should also be comforted by the thought that she will eventually "flourish in eternal vales." Thel counters that she feels like a fleeting cloud, easily disintegrated, and it makes her feel as if her life has no value. In answer, the Lily summons the Cloud to explain to Thel why his brief existence is not meaningless.

The Cloud appears as a golden being, who tells Thel that he does not mind appearing only briefly and then passing away, because he knows he will be going on to "tenfold life" and "raptures." It is natural that, once he has ceased to be a cloud, he will become rain, no longer visible in the sky, but contributing to the growth of flowers and other beautiful things. Thel says that she is not so lucky—she will only be food for worms. To this, the Cloud summons a worm to explain that even if we are food for worms, this does not make us valueless.

The worm that appears to Thel is "an infant," swaddled and cared for by the "matron" Clod of Clay. Clay explains that "we live not for ourselves." Clay represents the earth into which we are committed when we die: God, she says, loves even his worms and must care for them. She invites Thel into her "house," the underworld, to see what is there.

In the final section of the poem, Thel accepts this invitation and proceeds into the underworld. Here, she sees "the couches of the dead" and finds that this is a land of "sorrows" with no smiles to be found. Finding her own grave plot, she sits, only to hear a voice emerge from the "pit." This voice asks a series of frightening questions: why can we not close our ears to our own destruction? Why are we so easily beguiled by smiles and "poison"?

Terrified, Thel gets up and flees, not wanting to hear any more, and returns to "the vales of Har," the land of her innocence.