In the distant future, rising sea levels and extreme weather events have made land scarce. While wealthy members of the private sector look to space, the government turns to the sea, hoping to uncover mineral and oil deposits and establish underwater cities.

The U.S. Navy launches Project Amphitrite, informally known as “Mermaids of the Military,” in which female sailors undergo a series of increasingly invasive gene treatments and surgeries to adapt their bodies to living underwater. Recruits are told that these operations are reversible, but nobody opts for reversal because no one wants to be the first test case.

A nameless mermaid explains that women are better suited to deal with the cabin fever of submarine life, capable of resolving conflicts without resorting to violence. But she also postulates that women are more marketable: after all, they serve as PR pawns who can convince “drylanders” to settle underwater. She notes that geneticists retain or even accentuate the mermaids’ traditionally womanly features—such as their breasts—even though these body parts impede their effectiveness in the water. As the women spend more time underwater, they become detached from the world above and begin to resent the government and military officials—mostly men—who subjugate and cannot relate to them.

After one of the mermaids reports seeing an unidentified creature approach their submarine, the battalion goes off to find it. On the hunt, one mermaid—Seaman Metcalf—sees the creature and suddenly breaks formation, racing ahead of the group. When the rest of the crew recover Metcalf, she is badly wounded. When Metcalf returns to the base, she is cruelly detained on dry land by the human medical staff, despite being far along in her transformation and having completely adapted to the water.

The nameless mermaid searches for the unidentified creature on her own and is confronted by a beautiful mermaid—engineered by a “private island”—and her entourage. This beautiful mermaid informs the nameless Navy mermaid of her plans to stage an attack on the submarine, liberate the Navy women, and build a new world—by mermaids for mermaids—in the deep ocean where they can swim freely. She tells the Navy mermaid that she is going to hurt her. The Navy mermaid doesn't fight back.

The Navy mermaid awakens back on the submarine in the medical bay. She imagines that scientists will rebuild her from the injuries she sustained from the private-island mermaid and remove her now-unwanted legs. She pictures herself returning to the bottom of the sea, singing with the other liberated mermaids.