Dr. Deep and his assistant Alexander DuBrow were given an offer by a pair of zoo owners to capture a live mermaid for their zoo with the offer of a one million dollar reward. Despite his ethics and concerns over the creature's safety, Dr. D. agrees. After his nephew Billy goes swimming in search of the mermaid, he gets attacked by a hammerhead shark, only to be rescued by a young mermaid girl. While Billy communicates with the mermaid, they are captured in a net and brought aboard The Cassandra, Dr. D's ship, where the mermaid is kept in a giant fish tank, ready to be given to the zoo owners. Despite this, Billy and the Mermaid begin to get along with one another, with Billy feeling guilty over capturing her.
A few nights later, Billy and his sister Sheena awaken to see Dr. D unconscious and surrounded by masked men. They hope that Alexander can help, but it's soon revealed that Alexander was working for the masked men this entire time. It turns out that he was seeking the mermaid to give to his bosses for a twenty million dollar reward. Alexander and the masked men place the Deeps in the giant tank and throw it off the boat, while they escape with the mermaid in their own boat. But before the Deeps are done for, they are rescued by the other mermaids, who are out to save their young brethren.
They catch up to the kidnappers and try to save the mermaid, only for their dinghy to be set on fire. They manage to put the fire out as the kidnappers plan to throw them back into the water. Suddenly, the boat begins to rock as the mermaids are violently shaking it, eventually freeing the young mermaid. The Deeps escape, but manage to see that the young mermaid is safe. When the zoo owners return to Dr. D with the check for a million dollars, he turns it down, claiming that there were no mermaids. At the end of the book, Billy returns to the ocean in hopes of finding the mermaid, only to be attacked by a giant sea monster.
The Mermaid is basically similar in appearance to traditional variations of merfolk used in other fiction. She resembles a teenage human girl with long blonde hair from the waist up with a long fish tail in place of legs. Like certain mermaid depictions used in current media, she wears seaweed as a bra and sea shells as jewelry.
"Kruu" is the only thing the Mermaid says, and her favorite food is squids.