England, the turn of the 20th century. Archaeological scholar Professor Purwell, his blind daughter Diana, and young son Paul are travelling by steam train to the Vale of the White Horse. Diana is very keen to visit this ancient local chalk hill drawing. Purwell is to reside with his sponsor Sir George Mortenhurze at his mansion house, with the pair embarking on a project studying the true history behind the legends of King Arthur.
A horse and carriage arrives to collect the Purwells but as they travel the country roads, Diana suddenly becomes uneasy and asks that the carriage be stopped. She says that there is something strange nearby - indeed, there is a fine white horse standing in the adjacent field. It gallops away. Diana feels this was more than just any wild horse. Carriage driver Todman seems uneasy about Diana's demeanour.
On reaching Mortenhurze's manor, the Purwells are introduced to his daughter Estelle. In private, Todman and Mortenhurze discuss in low tones that Diana has sensed the mythical Moon Stallion. The two plan to capture the creature come full moon.
Estelle and Diana discuss the many old wives' tales told about the stallion and how it is her father's obsession. The stallion was last seen nine years ago and its appearance then coincided with her mother's death. The legends say that anyone who sees the stallion by full moon will die.
Purwell and Mortenhurze are in the nobleman's study, discussing questions such as the significance of the chalk horse, the possible defences of the area and the battle tactics of Arthur.
In the stableyard Estelle asks that Todman bring Duchess the prize mare to meet Diana. A recalcitrant Todman is unwilling, saying the horse is odd around strangers but Estelle insists. When the horse is brought before Diana, it rears up and whinnies in distress. Todman skilfully brings the beast under control. Appearing to say something in the horse's ear - could he be one of the fabled 'horse whisperers'?
That night, Purwell and his children discuss the pagan myths that surround the chalk horse and the local area - many are tied in with Epona the Moon Goddess, whose Roman analogue is a namesake to Purwell's daughter - Diana.
