It's not a missing episode, but a behind the scenes documentary about making The Daleks Invasion of Earth: 2150 has been discovered after being lost for 40 something years and can now be seen online
HEREDoc, you groovy hip cat, I'm digging the sixties vibe man! That's a real happening scene etc etc
Austin Powers-Speak aside, that is actually a fascinating insight into the artistic backdrop against which the Daleks and the wider show operated. Easy to see now why the show is so much part of our cultural DNA