'Spare no sympathy for those creatures. They were witches, they deserved to die.' A coach crashes on the M40. All the passengers are killed. The bodies carry no identification; they are wearing similar new clothes. And each has a suitcase full of banknotes. A country vet delivers a foal. The mare has a deep wound in her forehead. In the straw, the vet finds a tapered horn. In the darkening and doomed world known to its inhabitants as Tir na n-Og, the besieged humans defend the walls of their citadel Dinorben against mythical beasts and demons. The TARDIS’s link with the Eye of Harmony is becoming ever more tenuous and is in urgent need of repair. But the time machine takes the Doctor and Ace to a village in rural Wales, and a gateway to another world.