Adventure!

Lucy-Ann, Jack, Dinah, and Philip spend the holidays in a cottage near a strange, deserted castle, and find themselves in the midst of another adventure, involving a torch flashing at night and men at work in the depths of the castle.

A science fiction tale set under the ocean rather than in space, taking place in a future when whales are herded by submarines and the world is fed on plankton. The adventures of a former astronaut who now works in the deep ocean.