The story is told from the perspective of Rio, a girl from the underwater city of Atlantia.

Initially, Rio wants to go aboveground, but she has promised her Below-loving twin sister, Bay, to remain in Atlantia, an artificial city built entirely underwater.

In a panic, Rio accidentally revealed the siren voice she had hidden since birth to a priest, risking arrest, but she was able to leave without issue.

Rio searched desperately for an explanation for Bay's choice, eventually meeting her siren aunt, Maire, whom she last saw at her mother's funeral, and running away due to being engulfed in the memories of her late mother, Oceana.

After her encounter with Maire, Rio went to the swimming lanes motivated to grow strong enough to go through the floodgates and to earn enough money to buy a pressurized air tank, but she found herself to be far less skilled than Bay.

After swimming, Rio went to the deep market to search for True, where she told him she changed her mind about working together.

Nevio the Minister gave his sermon after the deep market closed early, leading Rio to discover his secret identity as a siren.

In a later discussion at Maire's apartment, Rio learned there had never been two sirens in a family until she was born.

The next day, it was announced that group burials would be held, since the death toll was too high.

Maire gave Rio a seashell with her mother's voice and gestured for her and True to escape.

Rio met Bay, Fen (True's best friend), and Ciro, the Above Minister.

Nevio opened the closet and dumped a dead Ciro and temple bat inside.

Hundreds of temple bats soared past Nevio to land on Rio.

Atlantia received reviews from the School Library Journal,[2] Justine Magazine, Salt Lake City Weekly,[3] and Shelf Awareness.

[4] Atlantia was also reviewed by multiple noted authors, including Shannon Hale, who wrote, "“Utterly captivating.