"Mission Of The Darians" *****
A massive miles long spacecraft holds the few remaining survivors of a dying civilization.
I like this one. I think perhaps I'm being a little generous with a 5 rating, but there is a fine story here, not badly told and some big ideas in play. It's not bad when you start out with one helluva kit bash representing a spacecraft tens of miles long.

Some nice model work going on here.
This episode reminded me of other stories I've read and watched: the short-lived series
The Starlost, David Gerrold's passed over Trek story (which he novelized) called
The Galactic Whirlpool, Star Trek's episode "For The World Is Hollow And I Have Touched The Sky" and TNG's episode "Up The Long Ladder."
1999's flying city is actually a worldship just in a different form.
There are a lot of decently thought out ideas in this story and criticizing it would be in the form of nitpicking. Suffice to say that overall I think this is the best
Space: 1999 episode I've seen to date.
"Dragon's Domain" ****
An Alpha crewman is haunted by a terrifying past.
I liked this, particularly with its bit of prehistory. We got to see some Earth politics at work as well as political lack of imagination. But the one BIG logic flaw not really addressed is how did the exact same collection of derelict ships find itself millions of light years from where Cellini had originally encountered them?
Nice touch having Russell reference them 877 days since leaving Earth. Thats almost two-and-a-half years at (we assume) relativistic flight. I like it also because it doesn't take the conceit that the past experiences have all happened within the framework of only one season or one year.