Amy and Dan are hot on the trail of the fourth Clue, but their ruthless relatives will stop at nothing to eliminate them from the competition. Their hunt leads them to an ancient city that has been a hot bed of Cahill activity for centuries. All that stands between the siblings and the Clue is a cryptic riddle, a missing relic, and the secret of their grandmother's past. The clock is ticking for Amy and Dan, but can they really solve a mystery that has eluded their ancestors for generations?

Rick Riordan is the author of the New York Times #1 best-selling The Lost Hero, the first book in the Heroes of Olympus series. Kids across the country voted him Author of the Year for that book through the Children's Book Council in association with Every Child a Reader. Rick also penned the New York Times #1 best-selling Percy Jackson and the Olympians series: Book One: The Lightning Thief, Book Two: The Sea of Monsters, Book Three: The Titan's Curse, Book Four: The Battle of the Labyrinth and Book Five: The Last Olympian. The first two books in his Kane Chronicles, based on Egyptian mythology, The Red Pyramid and The Throne of Fire, were New York Times #1 best sellers as well. Rick lives in San Antonio, Texas, with his wife and two sons.