Through his eyes the book explores themes of poverty and social injustice, as a deadly plague named the Blue Pox strikes The Mire, the poorest quarter of Emelan's capital city, Summersea.

One day while out running errands with his teacher, Rosethorn, Briar is summoned to the Mire to examine Flick, a poor friend affected by a strange illness.

The two bring the feverish girl to Urda's House, (a small charity hospital), where they learn that Flick is by far not the only one affected; many other people from the poorest parts of Summersea are ill as well.

They also learn, to Briar's dismay, that the hospital has been put under quarantine to help stop the "blue pox"—so named for the bluish sores that mark the skin—from spreading even further.

Briar, not wanting to lose his teacher, plunges after her into death, magically linking himself to the three girls and his beloved shakkan, or miniature tree.

Upon their return, they discover that Rosethorn has temporarily lost her ability to speak, due to the seizure and the accompanying block of oxygen flow to her brain.