The story opens with a bang - it looks as though Yii, the Stone Age boy, will be eaten by the tiger, distracted by picking up a remarkable stone. Amazingly he is transformed to a different age and the tiger is dead, but the Colonel who shot the tiger and the English soldiers in Victorian India fear he is a wolf boy and lock him in a cage at the house of the Colonel. Yii has extraordinary, repeating dreams.
Meanwhile Sarah in England, daughter of the Colonel, has become very strong and fit, having been running wild with her twin cousins, three years older than her. Expelled from a horrid boarding school she spends time in Oxford with her aunt and her aunt's husband, head of Sherborne College. She learns that there are priceless treasures hidden away during the Civil War (1650) and feels she should find them before a dodgy professor does. Sarah also has unusual, repeating dreams.
When Sarah goes to India to be with her parents, fascinated by the boy in the cage, she communicates with him, and starts to teach him to speak English. She suggests that it was the stone he picked up which somehow transported him to the new time in 1880s India, and it was the tiger that herded Yii to the rock face where he found the stone. The stone is beautiful, small with a five-pointed star on it, always conveying a mysterious atmosphere. They call it the Star Stone.
Sarah helps Yii escape from the cage. They go to the rock face where Yii was transformed, but to their horror Sarah is also transformed to Yii's time in the Stone Age. As she cannot get back to her time, Yii takes care of her and helps her to adapt. She has constant underlying fear, but the tribe accept her and she tries to be part of it.
After various adventures they attempt for the umpteenth time to get Sarah back to her time with the Star Stone. Again, they are both changed to another time, but not Sarah's. Yii helps a wounded Greek trader, who in turn helps them to plan a route to the west. Sarah and Yii discover that they have been having similar but complementary dreams for some time, and decide that these dreams are signs of a common destiny, which Sarah is sure must be to find that sacred treasure. They need to get back to England somehow, a difficult and possibly dangerous journey, but with great determination and boldness they set out, aided they hope by the wonderful Star Stone.