Ingrid Fleming lives with her mother Helga and uncle Magnus Colborne in a house on Dartmoor. They are visited by her cousin, Hugh Drapier, recently returned from an adventure in Tibet, where he came across two fellow Westerners who had just stolen a sacred stone (a flint-like rock which seems to be the broken half of a larger whole) from a monastery. Drapier was given the stone as he seemed the most likely to escape, on the unspoken condition that he return it if the two men make it back to England. But he has developed a fascination for the stone (a moving starscape or night sky can be seen in it), and cannot bring himself to relinquish it. He has premonitions of his own death.
When Drapier and Ingrid visit nearby Devil's Tor, lightning strikes the massive rock atop the Tor, uncovering an ancient tomb. Ingrid has a vision of the goddess she intuitively always knew was buried beneath the Tor's rock. Drapier ventures into the tomb, where he picks up what he thinks is the Tibetan half-stone, but which turns out to be its twin, sundered thousands of years ago. Fate is moving to bring the two halves together. However, the adventurers whom Drapier received the stone from arrive to claim it back.
The rejoining of the stones is merely a symbol for a greater union that Fate is bringing about: Ingrid is to be the mother of a saviour figure. The father, it turns out, is to be Henry Saltfleet, one of the adventurers. The novel ends when he and Ingrid finally admit this to themselves.