The first man on the moon was there on July 20th, 1969. After December of 1972, there has not been a person on the moon since. But, what if the missions to the moon had kept on going? By what time could we reasonably had a permanent habitat there. A quick search turns up a lot of questions that say that we could have had a moon base by now, given that the economic need was there. What if the cooperation between the Soviet Union and the United States on space flight hadn't happened? Surely the next step after a man on the moon would've included some sort of habitat there?
I'm not talking about full-scale cities and the likes, but a simple habitat like the one used in the The Martian movie. So basically a couple of modules/pods that are connected together to make an expandable habitat. And before anyone asks, no, I'm not intending to have my characters plant potatoes on the moon.
Eventually it would grow from the research habitat described above to a more permanent settlement there. There needs to be something to get there that's hard to get elsewhere. It might be Helium-3 for fusion purposes, or as a hub for space travel (go from Earth to the moon, then board a larger ship and continue on towards Mars, Venus or the outer planets).