Great scene:

Near the opening, they're tossing darts at balloons with Straker's face on them.

Would tossing darts in a room with a window be a good idea on the moon? In an interior room, yes, but an exterior room?

Actually, given the amount of pressure a porthole or "window" would have to hold in relative to the near-vacuum outside, it is highly unlikely a dart is a threat to moonbase. Try throwing a dart at a window with toughened glass here on Earth and you'll get the picture.

I wish they hadn't killed the alien at the end. Apart from thaty it was a good story. It was about the only episode where they showed an alien as a character and not merely a danger to be got rid of.

I think killing the alien at the end was the whole point of the episode. Personally I saw it as an allegory against racism.

Very good episode, but why would the alien shoot just one, single window? Moonbase didn't even know he was out there, and he could have taken pot-shots at every window, creating a panic. Damaging equipment outside, which could be life support or a reactor, seems like a good idea, too.
Had the alien been brought into Moonbase, I guess he would have removed his helmet, but wouldn't he need that liquid inside it to survive?

No, the aliens can breathe our atmosphere. The alien in "Square Triangle" did it, so did the one in "The Computer Affair". Besides that, the aliens harvest our organs, including lungs that are designed to add oxygen molecules to blood cells.

The SHADO space suits sure have quite an oxygen and power store - how long is Foster out there? A day? And why didn't the Moonmobile crew make a thorough search to find Foster or his remains? They seemed to shrug and go home after he didn't respond on the radio.

The SHADO space suits sure have quite an oxygen and power store - how long is Foster out there? A day?

Foster didn't survive on his own O₂ supply, he used the alien's liquid breather.

No, Foster was breathing air all the time. The alien tried to figure out why Foster's suit wasn't sustaining him, and discovered a severed air line. The alien glued a bridge into place, but for some reason, Foster's labored breathing continued. The two continued to sustain themselves on their own life support equipment.

It was in "Ordeal" - Foster dreaming of being abducted - that Foster was in an alien suit and breathing liquid.

I seem to recall, at the end, when they take Foster's helmet off, he has to cough out all the liquid he was breathing.

That was "Ordeal", the "abduction dream", not this one with Foster stuck out, for real, on the lunar surface where an alien finds him and they eventually become trusting of each other.

In this one, Foster spots the moonmobile, tells the alien to stay there, he'll go get help. He falls on the way, unable to stand any longer, and his companions find him. Since Foster's radio doesn't work, the only way the others can hear him is to put helmet against helmet. When the one hears the word "alien", he stops listening to Foster and goes off, shoots the alien, and Foster is in anguish that the alien he managed to befriend has been cut down.

The alien couldn't have given Foster any liquid to breath without reconfiguring Foster's space suit and having a complete additional supply of liquid. Also, the alien was out and about - his spacecraft had been destroyed, so he didn't have a spare that Foster could duck behind a rock or into a phone booth to change into!

D@mn. I could have sworn that that happened in "Survival". I don't think I ever saw "Ordeal".

A nit;
When Foster falls upon that huge lunar rock, he moves the entire rock (revealing it to be a flimsy prop). Also, every scene of Foster and the alien on the lunar surface reuses the same rocks. Since Foster was so near the other moonmobile, why didn't they find him (when the other astronauts returned to their moonmobile). They obviously didn't search too hard.

The ep was entertaining, but the "Enemy Mine" type story has been done so often on other shows, it's a bit tired.

I've heard that experiments have been done in which test subjects breathed an oxygenated liquid for several minutes. It has been theorized that a spacecraft pressurized with such a liquid, instead of a mixture of gases, would enable a spacecraft crew to withstand acceleration better. If the aliens used such a system, they would probably also use the same liquid in their space suits.

I did see a science programme in which a mouse was made to breathe liquid. I'm sure such a process could be adapted for humans. It's already established that the alien craft can travel faster than light. Presumably, the crews wouldn't necessarily be in suspended animation for a prolonged flight. Straker grossly underestated the distance of the alien planet when he said a UFO traveled "a billion" miles to get to Earth.

Sure took a long time for the air in that room to blow out.

Why wasn't there some kind of automatic shutter on the window?

Why didn't the room have a handheld oxygen supply? The guy who suffocated could have survived then.

So what happened to Lt. Ellis?

Straker says, "Racial prejudice burned itself out 5 years ago!"
Gosh, remember when that happened?

What was the alien thinking? His only chance for survival was as a prisoner at moonbase, so why carry both guns?

Paul's trying to tell them about the alien & I'm thinking, why can't he just point?

I'd have liked to have seen more of the aftermath on the Moon, after Foster made it clear the alien had saved his life... to look anew at the black-and-white (no offense to Mark Bradley) thinking about the aliens. Would have been a better ending than the senseless ending with Tina.

Bradley was given command of Moonbase after they thought Foster was dead. He's alive, so is the promotion off?

Anybody who ever saw the 'Star Trek' episode, 'City on the Edge of Forever', would be feeling a sense of dread and deja vu; the old 'Stay there. Don't move." request inevitably leads to a death. It was a tactical error on Foster's part in two ways; 1 - he could barely walk, so the astronauts could only find him flat on his face, in pain and exhausted, which makes it look like the alien attacked him. 2- appearing before the rescue team, side by side with the alien, he could have warned them off and motioned to them not to hurt the alien.
Pretty good writing, when you can feel sorry for a alien who didn't utter a single word.

I always felt the weird hissing, warbling sound of the Moon-Hopper's engines was strange and kinda alien-like. Design-wise, you can see it as a definite precursor to 'Space:1999's Eagles.

Treklon (waaaay back in 2005); 'The ep was entertaining, but the "Enemy Mine" type story has been done so often on other shows, it's a bit tired.'
Technically, it can only be 'tired' if you've seen 'Enemy Mine', and similar episodes of TNG and Enterprise, that used the same situation-- a human and an alien must cooperate, and THEN this episode of 'UFO'. Because, since this episode was made in 1970, long before other shows copied the idea, it's other shows that possess this 'tired' idea.

Treklon (waaaay back in 2005); 'The ep was entertaining, but the "Enemy Mine" type story has been done so often on other shows, it's a bit tired.'
Technically, it can only be 'tired' if you've seen 'Enemy Mine', and similar episodes of TNG and Enterprise, that used the same situation-- a human and an alien must cooperate, and THEN this episode of 'UFO'. Because, since this episode was made in 1970, long before other shows copied the idea, it's other shows that possess this 'tired' idea.

Of course, this concept was used before UFO came along.

Was it? I can't think of any obvious examples at the moment.

The 1958 movie, The Defiant Ones, uses this idea.

Well, that's one example in the 12 years between that movie and this episode.

You requested an example, I provided one.

End of story.

Not such a 'tired' concept by 1970, as I stated then. The vast majority of examples happened after this episode, as far as I'm concerned.
I remember an episode of 'The Six Million Dollar Man' in the 70's where Steve Austin meets a Japanese soldier on an island, who still thinks world war 2 is still happening. Not sure if they had to work together to survive, or if it was just a fluke encounter during an OSI mission.

Random thoughts:

* As someone mentioned, why go to all the trouble to sneak a ship to the moon just for what amounts to a harassing/nuisance attack?

* Must be an early episode, before Billington started wearing a hairpiece. His hair is thinning badly here.

* Straker addresses Foster as "commander" (?) Is that due to his command of moonbase?

* why does Foster take Straker's car, when his own (hideously colored) car is there in the studio parking lot?

* continuity error? - Foster takes Straker's car to his gf's apartment, is making out with her.....then we immediately cut to Straker driving his own car again.

* Foster is the moonbase commander here. Guess LT Ellis was demoted?

* I imagine Dinky didnt sell many "Moonmobile" diecasts. It may be functional but its just plain ugly. The Eagle in S:1999 is just as functional, yet really looks smashing to me - just an excellent design.

* The moonmobile's hovering is great for negotiating tough terrain, but the dust plumes sure give away its location to the enemy.

* The yellow helmets/etc don't help much there either.

* The rifles here are especially toy-looking.

* what is the alien holding in his hand at 31:31?

* interesting that LT Bradley wears a Skydiver mesh shirt off-duty at moonbase. Perhaps he served there previously.

* the "racial" scene doesnt work. Just doesnt.

* yeah, all Foster had to do was point.

* I do like the scenes that show that working for SHADO has severe personal-relationship costs.