The Doctor, Ian, Barbara, and Susan arrive on a spaceship and immediately worry about the human crew, who are being affected by telepathic interference from the Sensorites. Susan communicates with the Sensorites and discovers they fear a human attack and are only defending themselves. Journeying to the Sensorites' planet, the Doctor tries to find a cure for an illness affecting both the Sensorites and Ian, but learns it's the result of deliberate poisoning. The Sensorite City Administrator then complicates matters by attempting to discredit and frame the TARDIS crew.