A demon dragon known as the Caoránarch is on the loose in the City of Sorcery, killing indiscriminately and then disappearing without a trace.

Remi and her friends are certain the appearance of the Caoránarch has something to do with the corruption of the summer Fae, and their acquisition of the Twilight Horn, but as they investigate the origins of the demon dragon, their head instructor throws new obstacles into their path—including banishing them to dangerous departments not meant for students.

As they struggle to survive their new roles, the killings continue and a strange plague takes hold in the city. Overrun with problems, and struggling with their new departments, Remi and her friends make one last desperate play to save the city—or doom the world if they fail.