When a teleportation accident goes badly wrong, Nyssa finds herself stranded on the freezing slopes of the Swiss Alps in 1963. But is it mere coincidence that she finds shelter in a snowbound school haunted by a malevolent poltergeist? When the Doctor arrives, Nyssa and the other inhabitants of the school soon discover that the ghost is merely part of a darker, deeper and more deadly game involving rogue psi talents and something else... Something not of this Earth.