Mermaids have been sinking ships and drowning humans for centuries, and now the government is determined to put an end to the mermaid problem—by slaughtering all of them. Luce, a mermaid with exceptionally threatening abilities, becomes their number-one target, hunted as she flees down the coast toward San Francisco.

There she finds hundreds of mermaids living in exile under the docks of the bay. These are the Twice Lost: once-human girls lost first when a trauma turned them into mermaids, and lost a second time when they broke mermaid law and were rejected by their tribes. Luce is stunned when they elect her as their leader. But she won't be their queen. She'll be their general. And they will become the Twice Lost Army—because this is war.