In the final episode of the six-part story "The Sensorites," the Doctor (William Hartnell), still trapped in the 28th century, finds out who is really responsible for the disease that has been killing off the telepathic Sensorites. But neither the Doctor nor his companions are out of the woods yet -- nor, for that matter, are the Sensorites, whose planet again faces destruction. With time running out, the Doctor must pull one more trick from his sleeve to set things right. Written by Peter R. Newman, "A Desperate Venture" originally aired on August 1, 1964. ~ Hal Erickson, Rovi