The cover shows Captain Jack the Knife, with many gold coins in the air.
Violet Packer and her brother Shawn are visiting their Uncle Jim for the summer. He lives in an old house by the seashore. Jim was a sailor and he has collected strange and fascinating items from the sea. While exploring a back room, Violet and Shawn discover a locked trunk, which is a pirate's chest. The trunk is wrapped in old, heavy chains and is also locked with a huge rusted lock. But Violet and Shawn managed to get it open.
They lift the lid slowly and see that's filled with antique jack-in-the-boxes. One box is hidden under the others, covered in dust and it plays an odd, unpleasant song. Then up pops a mean-looking puppet, an old pirate with a dirty red bandanna over his long greasy hair, scars on his cheeks, a beard and one eye missing. Then he bounces on his spring, rasping "Thanks for letting Jack the Knife out!". Violet and Shawn now face new and troubling questions: Will Jack return them to their uncle? Just how much is a pirate's promise worth? 
Violet and Shawn Packer arrive at Sea Urchin Cove, they eat out at a restaurant called The Whistling Clam, when Violet told the waitress they were staying with their Uncle Admiral Jim, the waitress told them not to go there and go back home.
They arrive at their uncles house, where they see a black cat, but the cat starts talking, she was calling Admiral Jim. Jim appeared, and gave them a hug, they had crab cakes for dinner, he told the story of why his cat, Celeste, is able to talk.
There was a man named Danny Lubbins, his ship sank, and was stranded at sea with the black cat for 300 days, to keep himself from going mad, he taught the cat how to talk, they spotted land, but only Celeste made it, and Danny died.
Before they go to bed, they explore their uncles knickknacks and fake treasures, including one that was used on a movie set, but was left there, but then, Jim told Violet and Shawn to stay away from the forbidden room.
Slappy the Dummy interrupts the story, and asks the reader if they think the story is starting to get scary.
The next morning, after their lobster omelet breakfast, Jim leaves the house, and while gone, Violet and Shawn sneak into the forbidden room, they find a chest full of jack-in-the-boxes, each one played Pop! Goes the Weasel! as they turned each crank, Celeste came in, and told the children she was going to tell Admiral Jim on them, but since they were in trouble anyway, they opened the last one, but it wasn't playing the song the others were, it sounded eerie, like horror movie music, then a loud deafening explosion occurred.
Out popped Captain Jack the Knife and his crew, a female pirate named Mad Madeline, a chimpanzee named Chuckles, and a two headed pirate named Salty and Pepper Magee, the kids discovered that their uncle has turned into a jack-in-the-box, they all stepped out of their boxes and grew life size, the kids fled, but the pirates caught up, Jack told them his Canary named Captain Pip was trapped on Clam island, and told them he wants Violet, Shawn, and his crew to get the bird back, if they get the bird back, they would go back in their boxes and Jim would turn human again, as they got on the ship, The Jolly Sea Scab, Celeste whispered to Violet not to go that ship.
Slappy interrupts again to brag about Celeste.
The ship ride to clam island was a nightmare, Salty and Pepper kept arguing with each other, Madeline kept making bad jokes, and Chuckles was stirring the ship. Then all the sudden, the ship started sinking, and they lost their map, Chuckles took the only life boat, and Violet and Shawn plummeted into the icy cold ocean.
As hope seemed lost, Madeline, Salty and Pepper had a raft the whole time, they got on board, and Madeline attempted to shoot a flame pistol in the sky, but the pistol was wet, luckily, another pirate ship came, and it almost crashed them.
Slappy interrupts again, and says he hates it when ships do that.
This ship belonged to a pirate named Captain Billy Bottoms, but he rescued them so he can have them as slaves, working on the nearest island picking bananas. While there, Madeline gave a signal, and Violet and Shawn ran for it, but they landed in a banana cart filled with tarantulas, they got out and saw two girls about Violet's age, they told Violet and Shawn they were on Clam Island, it was called that because the island was shaped like a clam shell.
Violet and Shawn find the Canary, then they and the two pirates sneak on Billy Bottom's ship, who isn't there because he was still on the island looking for his prisoners, they escaped the island with the Canary.
When they arrived home, they gave the Canary to Jack the Knife, but he said it was the wrong bird, but he accepted it, but then, he told Violet and Shawn that he was not going to bring Jim back, he was going to leave him a toy forever, Violet said, "But you Promised!" Jack then said he gave a Pirates Promise, which is worth nothing.
As the pirates surrounded Violet and Shawn, the Ghost of Danny Lubbins came in the room, Jack the Knife began shaking in fear, he was afraid of ghosts, so Jack and his pirates shrunk to toy size, and went back in their boxes, and Jim turned human again, the ghost of Danny Lubbins took Celeste, and he thanked the children, then he vanished with the cat. When Jim wakes up, he talks to Violet and Shawn like the whole thing never happened, though it certainly did.
Slappy concludes the story, jokes about Jim, talks about how him and Jack the Knife have much in common, and wrapped up the story.
- The working title for this story was "Jack Attack".
- This is the second Goosebumps book to feature a pirate as the main antagonist. The first book to do this was Creep from the Deep.
- This title sounds similar to Attack of the Jack-O'-Lanterns.
- In one scene, Uncle Jim drank something that he claimed was Diet Sprite. Violet said that the liquid didn't look like Sprite because it was brown. This could possibly hint that it was an alcoholic beverage.
- In the book, Jack the Knife's hook was on his left hand, but on the cover, it's on his right hand.
- Jack the Knife's appearance on the cover looks nearly identical to the appearance of a character that appeared on the cover of the book Pirates of the Sea!. Both books were illustrated by Brandon Dorman.
- This book references Diet Sprite, Pirates of the Caribbean, Johnny Depp, and the movie Jaws.
- ↑ Amazon listing (November 29, 2016)
- ↑ FantasticFiction listing (November 29, 2016)
- ↑ An overcast day in Sag Harbor. I'm indoors, writing a new Goosebumps book called "Jack Attack." No, it's not about Jack Black.
|Goosebumps SlappyWorld series|
|#01 Slappy Birthday to You|
|#02 Attack of the Jack|
|#03 I Am Slappy's Evil Twin|
|#04 Please Do Not Feed the Weirdo|