The TARDIS lands on a space ship sometime in the 28th century. They are told that the ship is held by the Sensorites from the Sense-Sphere planet. The Sensorites communicate with Susan and the TARDIS crew go to the planet to talk with the Sensorites. The Sensorites state that they were visited before by humans and that after they left many of their people began to die. The Doctor discovers that the waters below their city had been poisoned by some of the human space travelers and works things out with the Sensorites. The captain of the space ship promises to not say anything of the Sensorites and their planet.