At some point before Violet and Shawn Packer arrived at the home of their Uncle Jim, he trapped Captain Jack the Knife and his crew in jack-in-the-boxes, and locked them in a trunk.
Violet and Shawn later release the captain from his jack-in-the-box, as curiosity gets the better of them. After being freed, Jack grows to his normal height, and soon transforms Violet and Shawn's uncle into a jack-in-the-box. He strikes a deal with the children, in which they will travel Clam Island with Jack's crew and retrieve his canary, Captain Pip. In return, Jack will turn their uncle into a human again.
After a long adventure, the children manage to return to Jack, though the latter claims that the canary they brought back is not Pip. While saying those words, the captain accepts the canary, and reveals to the children that his deal was a "pirate's promise," in other words, worthless.
As he and his pirates surround the kids with malicious looks on their faces, a ghost appears, causing the pirates to recklessly shrink and scramble back into their boxes.
He wears a red bandana over black hair, as well as a bright blue jacket over a black and white striped shirt. He has large brown eyes and a long nose above a black mustache. He carries a long-bladed knife in his right hand, and where his left hand should be a long curled hook replaces it.
Being a pirate, he is ruthless and fearsome. Not much else is known about him, however.
- His hook is on his right arm on the cover of the book, however the story describes Jack as having it on his left.
- The name Captain Jack the Knife is a likely reference to the song "Mack the Knife".