As Judy and Honey watch the stony-faced stranger choosing between a plastic and an ivory chess set, Judy has no idea that she is making the first move in a dangerous game with cold-blooded criminals. In fact, she doesn't even realize that there is a mystery—not until twelve hours have passed and still there is no word from her father. Why hasn't Dr. Bolton phoned? Where is he? With an emergency case? Or could the prison break have something to do with the doctor's disappearance? Judy finds a clue concealed in one of the twin knights with which she and the sinister "Seal" have been playing a game of chess. But this clue leads only to another puzzle. Why did "Seal" fly into a rage at her innocent question? Obviously, there is more to this chess game than can be seen on the chessboard. Peter, Honey, and Horace help Judy all they can. But it is Blackberry, Judy's strong-minded cat, who eventually shows her the secret in the barn. With this knowledge, Judy now knows how to move the pieces on the "chessboard," and in a thrilling final move, checkmates a gang of desperate criminals.