In her bedroom on board the TARDIS, Nyssa dreams of her long lost home, and a Forbidden Archway that never really existed — or did it?

In the TARDIS corridors she finds a flight of steps, a door marked ‘keep out’, and beyond, a locked room full of secrets. In particular a casket containing a jewel seems to call to her. “Nyssa of Traken, do you want to be my friend?”

So begins a journey that takes Nyssa from snow capped mountains to vast cities, and introduces her to people she seems to know so well. What dangerous landscape has she wondered into — and can she ever escape to safety, and the comfort of her travelling companions?