Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they've received an invitation to a mystery!   When their friend Sam gets a letter full of riddles from an eccentric millionaire, the Aldens are eager to help him crack the code. If Sam can solve the puzzle in time, he will inherit the millionaire's money! But Sam isn't the only one in town to receive such a letter—and whoever solves the mystery first will win the prize. The Aldens are eager to help their friend, but solving the millionaire's mystery turns out to be even harder than they expected. When every clue eventually leads them back to Sam, the Boxcar Children begin to wonder: Could the answer lie somewhere in Sam's mysterious past?