Plot: Reed Richards sends up a signal flare that literally says "The Fantastic Four", summoning the other members of the team, who are scattered all other town. Sue Storm runs around all invisible, Ben Grimm shows off how monstrous he is and gets shot at by cops, and Johnny becomes the Human Torch, accidentally melting a car and some military planes. And almost gets blown up by a heat seeking nuclear missile, because, um, apparently we just shoot nukes at anything that flies around. Reed uses his stretchiness to save Johnny.
After this rapid-fire introduction, we jump straight to the origin. Our four heroes are desperate to beat the Ruskies into Space, because 'Merica, so they illegally hop on a rocket and launch themselves into space. Bombarded by Cosmic Rays (YES!), our heroes crash to earth, mutate, nearly kill each other, and promptly decide to become Superheroes, because comics are awesome.
Nowadays, that would take about 5 issues... but we're still not done with the FIRST ISSUE! That's right, the story continues. Back at Reed's Apartment (?), the gang discovers that Atomic Power Plants across the world are being sucked underground into giant pits. Meanwhile, in French Africa, we see first hand a Power Plant being dragged underground by a Giant Monster (Yaaaaay!!!).
Tracing the vibrations, the FF fly on their Private Jet to MONSTER ISLAND! Immediately they are attacked by a Giant, Three-headed Monster, that Reed promptly lassos into the ocean. A sudden cave-in causes Reed and Johnny to fall into a hidden cave full of glowing diamonds, and the Mole Man is introduced.
Meanwhile, Sue is attacked by a Rock Monster. The Thing (also something of a Rock Monster) steps in and whoops the Rock Monster, cause the Thing is awesome. Meanwhile, the Mole Man explains his origin to Reed and Johnny (he was basically just an ugly guy who ran from society, got stuck underground, and somehow made a buncha giant monsters work for him. Man, it's lucky comics aren't real. We'd be up to our ears in Internet Trolls running around with Giant Monsters wrecking cities.) Anyway, the Mole Man plans on overrunning the surface world with his monsters. The Thing and Sue show up, and the Mole Man summons his biggest monster. There's a crazy scramble, the Mole Man is briefly captured, a bunch of crazy monsters show up, the Torch seals off the exit, the Four escape the Island, and the Mole Man blows everything up because Kirby ran out of pages.
Fun Stuff: Whew! That's a lotta crazy stuff for one first issue! Here's some Fun Stuff.
The Fantastic Four don't have costumes in this issue. They spend most of the issue in street clothes or explorer costumes.
The Human Torch looks quite a bit different in this issue. He's mostly just a swath of humanoid flames. He doesn't have any eyes or mouth in this issue. He also tends to melt everything he comes across. It's interesting how the FF are still learning their powers at this stage.
The Thing is not the pile of bricks at this stage. He kinda looks like a walking wall of clay. Maybe a little reptilian. He also kinda looks like an old school professional wrestler.
All the Invisible Woman can do at this stage is, um, turn Invisible. She's also the Invisible Girl, because Lee and Kirby weren't particularly enlightened back them.
The dialogue's not quite right yet. The humor and the humanity isn't there yet. Don't worry, it'll get there.
It's important to note that this was the first comic of the Marvel Age. Before this, Marvel was primarily doing Monster Comics (primarily by Kirby, of course), so in many ways, this comic is a Bridge Comic. There's lots of Giant Monsters, and, of course, the Thing is a monster himself (one of the Monsters from the earlier Marvel comics was called the Thing, in fact).
The Mole Man is defined by his insecurity. While the FF get turned into Freaks and immediately decide to use their powers for good, the Mole Man is just kinda ugly, and decides to hang out with Monsters and conquer humanity. Again, kinda reminds me of an internet Troll.
Kirby had a tendency for abrupt endings, and this issue has one of those. The King tended to run without a net, and the page count would sometimes run out before he was ready. On the bright side, he was a master of the rapid-fire wrap up, so it works out pretty okay.