One winter evening in a lonely Scottish inn, a small assortment of people are at supper when a brilliant light descends on the moors. A spaceship has landed and they find they are trapped inside an invisible wall which surrounds the house and garden. Nyah, the invader from Mars, is cold and ruthless and treats their efforts to escape with contempt. Her weird powers are demonstrated through Ghani, a monstrous robot which she controls. Finally, they have to decide which one of them will die to save the others, and possibly the world.