The full story
Reed Richards and his childhood friend Ben Grimm have spent years building a teleporter. It gets Reed recruited to the Baxter Foundation by its director, Professor Franklin Storm, to help build a bigger one, the Quantum Gate, with Storm's children Sue and Johnny and the Gate's original designer, Victor von Doom.
It works, and when they hear NASA will make the first trip, Reed, Johnny and Victor, with Ben, go through it themselves, to a dimension called Planet Zero. Victor touches a green, lava-like substance and the ground erupts. Sue pulls the others home, but Victor falls and is lost. The Gate explodes, and all four of them are changed. Reed can stretch, Sue can turn invisible and make force fields, Johnny can burst into flame and fly, and Ben is a creature of rock. The government takes them in to study them.
A year later, Reed is on the run, while Ben has become a military weapon and Sue and Johnny are working with the government too. Reed is caught and made to rebuild the Gate. The next expedition to Planet Zero finds Victor alive, fused to his suit, with telekinetic powers and the conviction that Earth must be destroyed to protect his new world. Calling himself Doom, he kills his way out of the base, Franklin Storm among the dead, and goes back through the Gate.
The four follow him. Doom has opened a portal that is consuming Earth into Planet Zero. They fight him together, Ben punches him into the portal's beam, which disintegrates him, and Johnny closes the portal. The government gives them a base of their own, and they call themselves the Fantastic Four.