Did Commander David Wheeler deliberately alter his ship's course to smash into and sink a mysterious vessel? His son, Richard, is convinced that he didn't. With friends Lucy and Peter, he sails out into the Bristol Channel to find the answer.

The three friends escape their pursuers, sail through a naval minefiled and encounter a mysterious unmanned ship. A house in a fishing village is also deserted - but the man they seek has left a clue in code.

The trio continue to evade their pursuers. They examine the map and decide to investgate Scudmoor - does the dark Gothic mansion hold the key to the Doombolt mystery?

Richard, Lucy and Peter escape with the Doombolt secret that will clear Commander Wheeler's name, but before they can deliver it, an unexpected encounter leads them up into the wild Brecon mountains.

As Richard, Lucy and Peter try to escape from Vallance in the wilds of the Welsh mountains, Bayard prepares to fire his experimental Doombolt. He is confident that the Navy will not be able to stop him...

The boys distract Vallard while Lucy makes a break for freedom. They are taken to Cragfest - a remote place surrounded by armed guards - and watch helplessly as Bayard sets up his Doombolt test.