A flare is seen in the sky above Central City; it reads, "The Fantastic Four!"
Susan Storm sees the signal and turns invisible, literally disappearing from a society party she was attending. She hops into a taxi, and hands the driver a bill while still invisible!
Ben Grimm is shopping at a mens clothing store when he gets the signal, and so he leaves by crashing through the wall, attracting the attention of the police who fire at him in a state of fear. He escapes through the sewers heading for an unknown destination.
Johnny Storm is working on his latest hot rod with a friend. He turns to flame and flies off through the air when he sees the signal. Johnny is pursued by fighter jets that think he's a "UFO". When they fire a heat-seeking missile at the him, he is saved by a long pair of arms that grab the missile and hurl it out to sea. The arms belong to Reed Richards, who we learn is the one who shot the flare in the first place. Once all four are gathered together, Reed fills them in on a fearful task that awaits them. but before that they reminisce about their origin.
So how did they become so fantastic? Well the story goes like this: To beat the Communists to space, scientist Reed Richards, sister and brother Sue and Johnny Storm, and pilot Ben Grimm secretly took off into space aboard a rocket. Along the way they passed through the cosmic radiation belt where the four were bombarded by cosmic rays. The ship returned to Earth on auto-pilot four crew members found themselves physically transformed, possessing remarkable new abilities. Sue could turn invisible, Johnny could turn to flame, Reed could stretch his body in amazing ways, and Ben was transformed into an orange, muscular "thing".
Taking the names Mr. Fantastic, Invisible Girl, Human Torch, and the Thing, they decided to use their abilities to become the super team known as the Fantastic Four!
Later, Reed shows the team black and white pictures of atomic plants in the Soviet Union, Australia and South America that have all been destroyed by cave-ins. Noting that Monster Isle is the equidistant center of the attacks, they travel there by private jet. Upon arrival, they're attacked by a giant three-headed monster but Reed saves them by flinging it into the ocean. Just then, A cave-in takes Reed and Johnny by surprise, and they drop beneath the surface. Meanwhile, Sue an the Thing are ambushed by a rock creature.
Far underground, Reed and Johnny encounter the Mole Man, who reveals that he is the one responsible for all of the power plant attacks. Mole Man tells them that he was a misfit; that he was ugly and abused by his fellow man, so he was forced to leave his civilization on the surface. He fell into a cave and became blind, but he discovered a whole new subterranean world which he would later come to rule!
Mole Man orders his monster army to destroy the Fantastic Four. Johnny distracts the biggest one while the team retreats through a tunnel which Johnny then seals shut behind them. After the Fantastic Four escape in their jet, the Mole Man unleashes a nuclear blast to destroy Monster Isle and seal off the underground world forever.
*note: the inker of this issue is unknown, although many believe George Klein is a likely possibility.