From #1 New York Times bestselling author Patricia C. Wrede, the fantastic conclusion to her tale of magic on the western frontier.

Eff is an unlucky thirteenth child...but also the seventh daughter in her family. Her twin brother, Lan, is a powerful double seventh son. Her life at the edge of the Great Barrier Spell is different from anyone else's that she knows.

When the government forms an expedition to map the Far West, Eff has the opportunity to travel farther than anyone in the world. With Lan, William, Professor Torgeson, Wash, and Professor Ochiba, Eff finds that nothing on the wild frontier is as they expected. There are strange findings in their research, a long prarie winter spent in too-close quarters, and more new species, magical and otherwise, dangerous and benign, than they ever expected to find. And then spring comes, and the explorers realize how tenuous life near the Great Barrier Spell may be if they don't find a way to stop a magical flood in a hurry. Eff's unique way of viewing magic has saved the settlers time and again, but this time all of Columbia is at stake if she should fail.