Space 1999 - Season Two Episode #23

An expedition to an Earth-like planet turns into a nightmare when Tony Verdeschi goes mad, for no apparent reason, kills another member of the crew, and disappears into the woods. The situation worsens as, inexplicably, the water and fruit turn to poison and the atmosphere changes causing all the metal in their ship and equipment to corrode,
effectively stranding them and cutting off all communication with Alpha. Alan and Koenig discover the corpse of another explorer who's records descibe an alien energy being on this planet, the sight of which can cause insanity. In order to save the lives of his colleagues, Koenig must find a way to communicate with this being.