The mystery of the Blue Admiral

Why would a thief, out of a whole room of valuables steal only an ugly, badly painted picture of an Admiral of the blue? Why would the eccentric old lady who lived in the house leave such an old, ugly picture hanging in a room full of beautiful antiques, silver and gold? And what had happened to the vast fortune the old lady's long-dead father had owned? The three Hadley children, Bobby, Peter and Eileen, take an immediate interest in this perplexing, intricate mystery.