The Sunken Desert was a location frequented by picnickers, which Missy turned into a bloody war zone. By manipulating the atmospheric temperance field, she manipulated the brainwaves of all visitors, who were complete strangers, into believing they were at war; she also "conveniently" left a small arsenal nearby, providing weapons for the oncoming hostilities.
After Missy "saved" Bertram from the war she created, he remembered the truth of why he'd come here. He would still have preferred rejoining the battle with his spear, were Missy not threatening him more directly to become her companion.
Around two days in, the Lumiat used a paradigm nebuliser to reverse what Missy had done. The visitors were horrified by their actions once they remembered the truth. (AUDIO: The Lumiat)