Ionna has never given up hope of reuniting with her lost son. A selkie who willingly gave up her seal skin for the love of a villager, Ionna ended up trapped between two worlds. Mist separates her husband's sleepy seaside village from the wide ocean, while a barrier keeps out danger from the mysterious woods that took her child. Ionna spends her days caring for her remaining children and waiting for the day she can cross into the woods to find her missing son-or meet the same unknown fate.

But now her sisters in the sea have come with a warning. Viking raiders are approaching their island, and they will do what men on boats do: pillage, rape, destroy. The protective mist barrier that encircles the island won't stop them. Ionna could return to the sea, but then she'll never find her missing son. Or she could save one of her other children with her selkie coat-an impossible choice.

During a desperate attempt to save all her children, Ionna encounters Bjorn, a Viking berserker haunted by all the lives he's taken. Hoping to pay his debts, he protects Ionna, then puts himself in her service. The two outcasts need to trust each other to escape the Viking leader Einar and his merciless warriors. Einar wants to make a new world-and he needs one of her children to do it. To survive, Ionna and Bjorn will have to overcome their respective traumas and create a safe space of their own.