Following the Dashwood family's move to Barton Cottage in the Devon archipelago, they are treated kindly by Sir John, who invites them to dine at his heavily fortified manor house on nearby Deadwind Island.

In this version of the story, Marianne falls into a rain-swollen creek and is attacked by an octopus, from which she is rescued by John Willoughby, a dashing adventurer and deep-sea diver who has come to the archipelago to visit his aunt.

To cheer up Elinor and Marianne, the two elder Dashwood sisters, Mrs. Jennings offers to take them to the undersea station, a massive iron and glass dome housing a large city, public gardens, shops, and a research laboratory where scientists plot new ways to defeat their aquatic enemies.

They eventually succeed with the help of a narwhal and other sea creatures; the Dashwood sisters and their friends barely manage to escape before the dome breaks and floods.

With the help of a special whistle that he brings, Elinor and Mrs. Jennings defend their boat by summoning a swarm of octopods, while the pirate chief is killed by the returning Colonel Brandon.

Sir John surmises that Lucy must be a sea witch – a monster that seduces human men and sucks the marrow from its victim's bones.

The island upon which Barton Cottage rests suddenly rises from the ocean but turns out to be a monstrous sea-beast known as Leviathan, awakened from a long slumber and hungry for all sorts of marine life.

Krueger states that readers should "have fun with Sea Monsters' incongruous, carnivalesque doppelgangers and then...revisit Sense and Sensibility" to learn more about Austen's characters.