Anything about this wad must inevitably start with the thing in the text file
The moon's orbit was slowed due to a headon collision with a giant ball of demons.
Escape the moon before it crashes into the Earth!
The first map is on the lunar surface. So there's lots of grey. An impressive sky texture is almost filled by the Earth, far closer than it should be, and causes all the rocks and buildings to cast shadows. Also there is a prominently displayed American flag and a bunch of pictures of the moon stuck on the ground for some reason. It's a fairly gentle introduction level, although there are quite a lot of small monsters and a booby-trapped chaingun.
The second takes us under into a base for space weapons research; specifically a "space plasma gun" and a "space rocket launcher". Quite tricky due to large monsters and low health but doable once you have learned where to run when the traps go off. The texturing uses E1 base textures, but in noticably unusual colours.
For the third we go back out on the surface into an area named Rocket Zone. This is both because you start with a rocket launcher and there are a bunch of actual giant moon rockets that protrude quite high above the landscape (cf. the Emerald Tower in the eponymous level from EARTH.WAD) This map is basically in four parts, the first, a sort of open base area, a battle around a rocket, a valley that traps you in with a lot of teleporting monsters, and a final open-sky area with more huge space rockets and buildings.
The fourth map is a tour around a computer station, a bit like the second but larger and more grey. It's not as hard as it looks. Just don't fire your weapons before you look behind you.
And finally, the fifth map is an arena-style battle. Huge numbers on the wall count down from ten to zero as more areas open up and let monsters in.
The only other thing I've got to say is that, like Vanguard, the difficulty curve is strange. The third map was easily the hardest and took me the most attempts to finish. The last map, despite ostensibly being a slaughter map, I beat first time. Although it's a huge fight, half the monsters kill each other and there's oodles of health and stuff lying around. For instance I hardly saw the two cyberdemons, everything else killed them before I had to. (Not that this is a bad thing, I love giant monster fights.)
Anyway yeah play this thing, it's good.