When the Enterprise's landing party arrived on the planet, the image of Losira appeared a number of times and each time killed a member of the crew by touching them. The victim would die from cellular disruption. She appeared in main engineering and fused the matter/anti-matter bypass controls, causing a runaway condition which, uncorrected, would destroy the ship.
When Losira's image appeared to the landing party, it would say "I... am for you." and try to touch one of the crewmen. They discovered that there were many Losira's, and that only the one "made for" a crewman specifically could harm that crewman. They created a strategy so that if Losira tried to touch one of them, the other members of the landing party would block her, as her powers would not affect them. The landing party questioned Losira, asking why she wished to kill them, and she seemed confused, and stated she did not wish to kill, and that she was "sent".
A tricorder signal led the landing party to an entrance to the underground part of the facility, where they discovered a large, cube-shaped computer and encountered several copies of Losira – a strategy designed to thwart their ability to shield each other from the deadly touch. Not even shifting positions could protect the landing party. The landing party seemed doomed when Spock materialized, and Kirk ordered him to destroy the computer. A security officer with him did this, and the copies vanished, the power to create them presumably destroyed.
A recording played, in which Losira revealed that she was the last survivor of the outpost, and that physicians had discovered too late what killed them: in creating the outpost planet they also accidentally created a deadly organism. Because she did not expect help from home to arrive in time, she set the output controls on automatic, to selectively defend against all lifeforms except Kalandans. McCoy theorized that ships returning home carried the epidemic, which destroyed the entire Kalandan race. Kirk believed the computer projected too much of the original Losira's personality into the replicas, and that they hesitated because they felt some degree of guilt at killing. (TOS: "That Which Survives")