to Sense-Sphere, the party learns that the previous visitors from Earth exploited Sense-Sphere for its wealth; half of them stole the spacecraft, which exploded on take-off. The Sensorite Council is divided over the issue of inviting the party to Sense-Sphere: some members plot to kill them on arrival, but others believe that the humans can help with the disease that is currently killing many Sensorites. When their first plot is foiled by other Sensorites, they continue to plot in secret. John's mind is eventually unblocked. Ian contracts the disease that has blighted the Sensorites, and told that he will soon die; it is revealed that he was actually poisoned by drinking water from the aqueduct. The Doctor finds the aqueduct and creates a cure for Ian. The plotting Sensorites capture and then impersonate a Sensorite leader, the Second Elder (Bartlett Mullins). Meanwhile, investigating the aqueduct, the Doctor finds strange noises and darkness. He sees and removes deadly nightshade, the cause of the poisoning, but while returning meets an unseen monster. Susan and Ian find him unconscious but unharmed. On recovering, he tells of his suspicion that some Sensorites are plotting to kill them. The plotting Sensorites kill the Second Elder and one of them replaces him in his position. John tells the others that he knows the lead plotter, but he is now too powerful, so the Doctor and Ian go down to the aqueduct to find the poisoners. Elsewhere, a mysterious assailant abducts Carol; Susan, John, and Barbara Wright (Jacqueline Hill) eventually find and release her. On discovering the tampered tools, they go into the aqueduct to rescue the Doctor and Ian. The leader discovers the plotters a little while later. Ian and the Doctor discover that the monsters were actually the survivors of the previous Earth mission, and they had been poisoning the Sensorites. Their deranged Commander (John Bailey) leads them to the surface, where they are arrested by the Sensorites. The Doctor and his party return to the city, pleading clemency for the poisoners. The leader of the Sensorites agrees and sends them back with Maitland, John, and Carol to Earth for treatment. Production Writing and casting Peter R. Newman developed the concept for The Sensorites in January 1964 and was officially commissioned to write the serial on 25 February by script editor David Whitaker. Newman's story was inspired by 1950s films set during World War II,