Journeying across different lands, the young mender Tanaquil and her familiar, a quarrelsome talking peeve, learn of the empress Veriam, who wishes to conquer from one sea to the next. Tanaquil is shocked to learn that the woman called 'Conqueror' and 'Child-Eater' is in fact her half sister, Lizra.

Remembering the powerful effect the black unicorn had on her people, Lizra has constructed a tremendous mechanical unicorn of gold as a symbol of her conquest.

The only problem is that it doesn't work - and Lizra commands Tanaquil to make the steam-powered unicorn move. Now Tanaquil must choose between assisting in brutal conquest or risking the ire of her powerful sister.