When Earth's entire infrastructure hinges on a single T-Mat transport system, a catastrophic malfunction at the lunar control center threatens civilization itself. The Doctor, Jamie, and Zoe discover that only one man holds the key to preventing global collapse. A gripping classic from the Second Doctor era featuring Patrick Troughton, Louise Pajo, and John Witty. Directed by Michael Ferguson with scripts by Brian Hayles and Terrance Dicks.