Mission Atomic (the 39 Clues Doublecross #4) by Sarwat Chadda

The thrilling conclusion to 39 Clues Part VI: Doublecross. A Cahill family member from the past is bent on revenge!CHECKMATEThirteen-year-old Dan Cahill and his older sister, Amy, are running out of time. An exiled Cahill known as the Outcast has already re-created three of history's worst disasters, and he's saved the worst for last. If Dan and Amy can't find and stop the Outcast fast, he will initiate a full-scale nuclear meltdown.But as Dan and Amy race around the world, they discover something horrifying. The Outcast's disasters are only a smoke screen to cover up his true plan, a diabolical revenge on the family that betrayed him. Soon Amy andDan will have to confront a hard truth: Sometimes the only way to save the world is to sacrifice everything you love.