What Bridget and Tom find themselves engaged in - involuntarily at first - is a search for something beyond themselves, something 'out there' which beckons to them:
like the mountain which Bridget thinks she can see from the beach on the horizon
like the mystery of the strange leaning stone on the headland, which seems to be pointing out to sea
like the mystery surrounding the tales of the eccentric Manny Reeve, who was mocked by everyone for saying he had 'found another world'
like the mystery of the bizarre book shop run by Stitgoe and Lulby, full of charts and documents and maps illustrating unheard of countries and continents
like the mystery of the sometimes ludicrous questions posed in the form of riddles that make up Retsinis' 'competition for clever young people'
and then later, the mysterious coded riddles left for them by the elusive Kalmom...
It's all a mystery, it's all a riddle - a challenge to go out and explore further...