The Alden family rents a houseboat to drift down the river, and encounters a mystery in the little towns along the river, involving a fast black car.

Following clues provided by the secretive Reddimus Society, the four Alden siblings travel to a dingo sanctuary in Australia and to Shakleton's hut in Antarctica to deliver the fifth and sixth artifacts, and then they work to unmask the traitor traveling with them.

After finding a statue in an old trunk, the children travel across the country with the statue and six mysterious boxes in an attempt to make sure the treasures all stay where they belong.

After solving riddles from the secretive Reddimus Society to figure out their destination, the four Alden siblings fly to China, where they deliver an artifact at the Great Wall, but when they are next rushed to an elephant sanctuary in Thailand, the Aldens suspect they have been tricked and take extra precautions against their enemies.

The Alden children travel on a Mississippi paddle-wheel steamer to visit an old family friend in his cabin near Hannibal, Missouri, and try to discover who is responsible for the mysterious activities near the house.