The new story in this issue featured the Watcher, so the reprint in the back went back to the introduction of the Watcher in FANTASTIC FOUR #13 (1963), a 22-page story inked by Steve Ditko.
In this story, Reed re-news his determination to beat the reds to the moon (and Roy Thomas helpfully points out this was before the mid-1970s detente), and discovers a new energy source from a meteorite sample. Planning to go alone, Ben convinces him otherwise.
I love these kinds of scenes, Ben stuffing Reed in a giant tube until he agrees to take them all on the mission.
Leaving exactly when the FF do is Ivan Kragoff, Russian scientist, who is going to the moon with a trio of trained apes. He's also going with no shielding on this ship, hoping to replicate the FF's cosmic ray experience from back in #1 and get powers. That actually works, and he and the apes get a variety of powers. The FF reach the moon first (with a glorious 1/2 page shot of the long dead city in the Blue Area), and face off against Kragoff, now known as the Red Ghost, and encounter the Watcher, who would of course become an important part of the FF mythos in later years. It's almost surprising what a throw-away concept he is in this first story.
As usual at this point in FF history, the story is pretty creative, gradually getting better and more complex. The art is wonderful, with lots of innovative ideas and clever bits of storytelling thrown in. Ditko inks over Kirby are always interesting to see (this in one of just a handful of examples on the super-hero books). He brings a lot of his own texture to it, but seems to be very faithful to the pencils as well.