From the back cover:

The race is on to find 39 Clues that safeguard a great power, and fourteen-year-old Amy Cahill and her younger brother, Dan, are shocked to find themselves in the lead. The search seems to be taking them to Vienna, and they hold a coded piece of Mozart's sheet music that's key to finding the next Clue. But tailed by a pack of power-hungry relatives, Amy and Dan can't see if they are sailing toward victory - or straight into a deadly trap.