The TARDIS arrives on a peaceful-looking planet, which will allow Nyssa to rest and recover from her experiences in the previous story, 'Four to Doomsday'. While she does so, the Doctor, Adric and Tegan set out to explore the jungle around the ship. And it is more complicated than it looks: strange wind chimes, natives who don't speak, and an Earth exploration party who have been vanishing one-by-one are just the surface of a far more complex problem that lurks inside...