The illustration of the cover shows a hammerhead shark about to attack Billy from underwater.
Billy and Sheena Deep are spending their summer vacation on the Caribbean island of Ilandra with their uncle named Dr. Deep who goes simply by Dr. D. While snorkeling, Billy is nearly attacked by a Sea Monster and manages to get away. Dr. D's assistant, a muscular graduate student named Alexander DuBrow, serves up lunch to the children: chicken salad sandwiches.
Only his cooking's terrible, so Dr. D offers to grill some fish for dinner. Billy overhears a private conversation between Dr. D and representatives for the Marina Zoo. The zoo crew wants Dr. D to find and trap a mermaid that has been reported in the area. The zoo offers Dr. D the princely sum of one million dollars.
Billy is too excited to sleep, so he goes out early the next morning at 6:30 AM to search for the mermaid. Billy's night swimming is suddenly interrupted by a hammerhead shark that almost kills him. Fortunately, the mermaid comes and attacks the shark, saving Billy's life. Dr. D nets up both Billy and the mermaid and drags them onto the deck of the ship. Billy tries to argue that the mermaid saved his life. But the mermaid is put into a large glass tank on the deck until the zoo crew arrives. Billy is enjoying dreams of Sea People when his slumber is interrupted by the sound of crashes and a splash. Above deck, he discovers the mermaid is in the process of being kidnapped! The men brutally toss Billy into the tank. Billy starts yelling for Alexander as Dr. D is knocked out on the floor and Sheena is offering no help against the four burly men. Finally, Alexander does show up only to side with the thugs.
The thugs toss Sheena and Dr. D into the tank, lock it, and then throw it into the water. The top of the tank is meshed, so the three characters inside the tank fight to break the locks before the tank submerges completely underwater. As they struggle, the "kidnappers" and Alexander speed off with the mermaid in tow. Just when things look their worst, a myriad of mermaids appear and hoist the tank above the water level. Dr. D and the children break open the latches and escape to the dinghy. As the other mermaids rescue her, the thugs attempt to set fire to Dr. D's dinghy. The mermaid is set free. Billy sneaks out by the coral again in hopes of catching the mermaid to say goodbye.
When Billy is searching for the mermaid, he is attacked by the Sea Monster which he encountered in the beginning of the novel.
Cover Art Gallery
- This book was never adapted for an episode of the Goosebumps TV series, though its sequel was. However, when adapted for the television episode, the sequel was simply retitled "Deep Trouble", which can lead to confusion to the viewer who could would expect it to be adapted from the original book instead of its sequel.
- Billy and Sheena's father was inadvertently named in the book. Billy is William Deep Jr., so that makes the father William Deep Sr.
- This story was adapted as a comic book in the Goosebumps Graphix book, Terror Trips drawn by Amy Kim Ganter
- This book was one of the three books to win a Nickelodeon Kids Choice Award, it won in 1995, the other two being Even More Tales to Give You Goosebumps which won in 1996, and the third being Deep Trouble II which won in 1998.
- The original tagline and cover of the book is a reference to Jaws.
- In the Goosebumps film, the sea monster was supposed to make an appearance as one of the monsters released by Slappy. It was going to have its own scene were its tentacles blast out from the lockers and attack Zach and Hannah, but were replaced by E. Ville Creeper Plants from You're Plant Food!. In its concept art, it's tentacles are very big and long, with a dark green tone. It also has four red eyes.