Mimi and Papa speed away to London, England in their little red and white airplane, The Mystery Girl. Tagging along are grandchildren Christina, 10, and Grant, 7. Mimi, a children's mystery book writer, has an appointment with the Queen! Papa is headed for the Rosetta Stone. The kids are supposed to go to the Tower of London and "behave themselves." But, uh-oh, Grant spots a big problem with Big Ben...and he and his sister, along with their two new best friends, George and Maggie, take a whirlwind tour from the top of the Eye, up the Thames River, down into a dungeon, and more -- in search of clues to solve The Mystery of Big Ben!