At some point before Violet and Shawn Packer arrived at their relative's home, that relative trapped Captain Jack the Knife and his crew in jack-in-the-boxes, and locked them in a trunk.
Violet and Shawn Packer later release the captain from his jack-in-the-box, when they manage to open the trunk. After being freed, Jack grows to his normal height. Afterward, Captain Jack the Knife turns their uncle, Jim, into a jack-in-the-box as well. He then makes a deal with the children, in which they will travel Clam Island with his crew and get his canary, Captain Pip, back. In return, Jack will turn their Uncle Jim human again.
After a long adventure, the children return, but Jack tells them they brought back the wrong canary, but he accepted it, he then told Violet that his deal was a Pirate's promise, and those are worthless, so he won't bring Jim back.
As he and his pirates surround the kids, the ghost of Danny Lubbins came, Jack is afraid of ghosts, so he and his pirates shrink and go back in their boxes.
He wears a red bandanna over his black hair. A bright blue jacket over a black and white stripped 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 his left hand is missing, just a long curled hook were his hand should be.
He is rude, a liar, and ruthless. He can be quite fearsome too. However, he is afraid of ghosts. When he sees one, he surrenders.
- His hook is on his right hand on the cover of the book, however the story describes Jack as having it on his left.