On a trip to Pennsylvania, presidential stepdaughter KC Corcoran and her friend Marshall are exploring the rest of the train when the special caboose at the back disappears--along with the Vice President.
Dink, Josh, and Ruth Rose investigate the theft of a great deal of money from the belly of a nearly life-size Tyrannosaurus Rex model that their friend, Jud Wheat, is using to raise money for a dinosaur museum.
Ruth Rose, Josh, and Dink attend a concert during which someone steals the star's diamond pendant, and they are soon on the trail of the thief. X is for X-RAY - Dink, Josh, and Ruth Rose are having an excellent time at the Penelope Gwinn concert. But during intermission, Dink injures his arm! At the same time, the singer's diamond necklace is stolen. Could the two mishaps somehow be connected? And could Dink's X-ray be a clue? It's up to the kids...
When they discover that one of the thumbs has been broken off the statue in the Lincoln Memorial, KC and Marshall set out to learn what happened and restore the thumb before a ceremony honoring the sculptor.