Storm Warning is the ninth book in The 39 Clues series. It is written by Linda Sue Park and was released on May 25, 2010. The geographical coordinates that appear on the ship on the book's cover (18 degrees, 15 minutes north; 77 degrees, 30 minutes west) indicate a point about six kilometers southeast of Albert Town, Jamaica.
As they were leaving China, Amy and Dan get a call from the Holts telling them that they knew where they were going. So Dan and Amy assume Nellie Gomez, their au pair, told the Holts.
On the plane, Dan and Amy get Nellie to give them some information. It turns out it wasn't Beatrice who hired her, Grace did. Nellie also tells them that she works for McIntyre, but does not tell them that she really works for the Man in Black.
As events in the Caribbean take place, Amy and Dan watch as the clue hunt kills a non-Cahill named Lester. Angry, Amy and Dan decide to face Aunt Beatrice, but Nellie kidnaps them, and take them to Moore Town to meet the Man in Black. They are then forced to solve a puzzle box that Amy and Dan had found in the museum that Lester had worked at. After solving Anne Bonny's puzzle box, getting the Madrigal clue of Mace, and the knowledge that they should go to England, Dan and Amy learn that Man in Black is Fiske Cahill, Grace's brother. He ran away as a kid, and that's why Amy and Dan never heard of him.
He tells them Madeleine Cahill's story: Gideon Cahill was trying to find the cure for the plague, and he did, but Gideon didn't know that the cure (a serum) altered the user's DNA, giving the users greater abilities in every area of human endeavor. Gideon gave each of his four children a part (only ONE part) of the master serum. Soon afterwards, Gideon died in a fire. The children blamed each other, and separated to start their individual branch. But no one else knew that Gideon's wife, Olivia, was pregnant when Gideon died and her family was separated. Pregnant with Madeleine Cahill, founder of the Madrigals, Olivia raised Madeleine to believe nothing was more important than family. So that is the Madrigals' goal- to reunite the family members. Fiske Cahill also tells them that the Lucians framed Arthur and Hope for murder in South Africa. Hired by Grace, Nellie was spying for Fiske so the branch would know about Amy and Dan in order to grant them a Madrigal status. Before leaving, Fiske reveals this to Amy and Dan. For the first time ever, the Madrigals give active status to someone not born in the bloodline: Nellie Gomez. Amy and Dan also become members. Amy and Dan are then given seven of the other Madrigal clues and head to England to search for the next clue, in the concluding volume of the bestselling series, Into the Gauntlet.