David, to his horror, saw the wire strands of the hawser slowly parting. He shouted a warning. Rose stepped forwards, slipped on the mud and rolled down the bank. David dived in after her as she fell into the water, just as a great brown wave of the biggest tide ever swept up the estuary towards them. David Morton and the twins are helping Rose and her brother Paul solve the mystery of their father's disappearance in his yacht Sea Witch. Somewhere in the mystery is the sinister picture-dealer Simon Donald. And what role is the red-headed girl Molly playing? And is the reporter James Wilson on their side or not? To cap it all, the Lone Piners find their enemies in this thrilling adventure are not only people - there is also the terrifying sea...