When a teleportation accident goes terribly wrong, Nyssa finds herself stranded within the freezing slopes of the Swiss Alps in 1963. But could it be simple coincidence that she discovers shelter within a snowbound school haunted with a malevolent poltergeist? When the Doctor arrives, Nyssa and the other inhabitants of the school soon find that the ghost is only part of a darker, deeper and even more deadly game involving rogue psi abilities and something else… Something not really of this Earth.