Iona, a Selkie, was brought forth from the ocean to protect seals from the evils of men. It's a compulsion she cannot deny, and she's lived a solitary life defined by her duty.

Cian is a Kelpie, a creature that can shift between a man and a horse. Born of the river, he must seduce and drown the women he loves in order to survive as he is- an immortal master of enchanted disguise. The two are trapped in a cycle of instinct and death, villains to the outside world and strangers to themselves.

Their fates become intertwined when Cian saves Iona's life after she's been shot by seal hunters. Cian vows to help find Iona's lost coat and claim vengeance upon the men who murdered her kin and gravely wounded her. The two creatures couldn't be more different from one another, but while journeying together they glimpse a different kind of life neither dared hope for themselves.

When Cian falls for Iona, the river demands he sacrifice her or else go mad and lose his magic. He refuses, and Iona finds him on the brink of insanity. She must navigate the human world to save him, even if it means losing everything she loves, including a new life with Cian.