Missionaries are always kept in groups of at least two of the same gender (called 'companions'), occasionally three if there is an uneven number in the area that they proselyte in. They have a rigorous schedule laid out for them, which includes: waking up early, exercising, studying the scriptures (individually and together), and so on throughout the day. This story happened while the two missionaries were studying the scriptures together. Zone leaders are missionaries put in charge of other missionaries. Similar to Jesus exorcising spirits in the New Testament when the missionaries felt possessed the zone leaders commanded the spirit to go away.
Mary Martin-Djurich (Mon,) studied this question.