The full story
Reed Richards is a brilliant scientist with no money. He persuades Victor von Doom to lend him his space station to study a cloud of cosmic energy, bringing his best friend Ben Grimm, Victor's head of genetics (and Reed's ex) Sue Storm, and her reckless brother Johnny. The storm arrives ahead of schedule, and it hits all of them.
Back on Earth, each of them changes. Reed can stretch like rubber, Sue can turn invisible and throw force fields, Johnny can burst into flame and fly, and Ben becomes a huge orange creature made of rock. After they save people from a pileup on the Brooklyn Bridge, the press calls them the Fantastic Four. Ben's fiancée can't face him and leaves. Reed sets out to build a machine to reverse what happened, while Johnny loves the fame and names them all: Mister Fantastic, the Invisible Woman, the Human Torch and the Thing.
Victor's company falls apart after the failed mission, and he's changing too, into organic metal that channels electricity. He blames Reed. He turns Ben against him, then powers Reed's machine himself to turn Ben human again, which speeds up his own change. He kidnaps Reed and fires a heat-seeking missile at Johnny. Ben uses the machine to turn himself back into the Thing, so he can help.
Now calling himself Doom, Victor fights the four of them in the street. Johnny and Sue heat him up while Reed and Ben douse him with cold water, and the thermal shock freezes him solid. Ben has found someone who loves him as he is, the blind artist Alicia Masters. Reed proposes to Sue, and the frozen Doom is shipped home to Latveria.