One panel of a priceless Antwerp altar piece was missing. Though the thief had confessed his guilt just before his death, there was only an enigmatic riddle to help the Bracome children discover its whereabouts. Peter, Michael and Jill, traveling with their parents in their little boat, the , set out to recover the altar piece. A cave-in in an underground labyrinth nearly ends their search. Some of the plot devices are melodramatic. It's a no-holds-barred derring-do adventure story of three British youngsters on a chase, with some effective cliff-hanging before success.