Amy & 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 & the Clue is a cryptic riddle a missing relic & the secret of their grandmothers past. The clock is ticking for Amy & Dan but can they really solve a mystery that has eluded their ancestors for generations?