A year ago, the March Kingdoms were at peace, and the Eddon family held the throne. King Olin Eddon is a prisoner in a faraway land, and Olin's heir Kendrick is dead—slain by treachery and dark, bloody magic.

With their father and brother taken from them, the royal twins Barrick and Briony have done their best to hold the kingdom together, but now Barrick has been captured in a failed war against the immortal Twilight People and Briony has been forced to flee.

Behind the Shadowline, Prince Barrick is lost and spellbound in the land of the fairies, while Princess Briony finds both allies and deadly enemies in unexpected places far from the land of her birth.

During their desperate journeys the twins discover that even a land's rulers may know little of it's true history, and that both families and nations can hide dark and terrible secrets.

But even if Barrick and Briony survive learning the astonishing truths at the heart of their own family and of Southmarch itself, they must still find a way to do the impossible: they must reclaim their kingdom and rescue their home and people from the multitude of powerful enemies—from traitors, tyrants, a god-king, and even the angry gods themselves.