Doctor Doom is the main antagonist of Fantastic Four and Fantastic Four: Rise of the Silver Surfer.
In the first film, Dr Doom is the main antagonist. Victor Von Doom was the CEO of Von Doom Industries. Although initially reluctant, he was convinced by Reed Richards to allow him access to his space station to test the effects of exposure of a cosmic cloud. However, due to Reed miscalculating, the clouds materialized ahead of time causing Victor, Reed, Johnny Storm, Sue Storm, and Ben Grimm to be exposed to them, giving each their own type of power. Victor gained the ability to shoot bolts of electricity, and transforming into organic metal (along with a scar on his face from an exploding control console).
Victor lost his company as a result, and the other four, who came to be known as the Fantastic Four, became famous for heroic actions and overshadowed the news of his company's downfall. He killed stockholder Ned Cecil and decided to get revenge on Reed, whom he blamed for everything.
Victor created a machine that used the cosmic waves, and convinced Ben (who was transformed into a rocklike creature) that Reed wasn't going to turn him back to normal as he said he would, thus driving them apart. He then used his device to turn Ben to normal, while simultaneously absorbing power from it. He became known as Doctor Doom, putting on a metal mask to hide his disfigured face.
Doom kidnapped Reed and tortured him by using the supercooling unit, as Reed's power was to stretch like rubber. He tries to kill Johnny with a heatseeking missile, as Johnny could start on fire, but Johnny avoided this by tricking the missile. Ben used the machine to turn back into "The Thing", and the Fantastic Four fought Doom, ending with Johnny burning him with supernova temperature (contained by Susan with a forcefield) and frozen in place when Ben sprayed him with water. He was sent back to New Latveria, presumed dead but electromagnetic interference suggested that he was still alive.
Fantastic Four: Rise of the Silver Surfer
In the second film, Dr Doom returns as the main antagonist. Doom was released from his frozen imprisonment due to the cosmic energy of the Silver Surfer. Doom tracked him down and asked him to join forces with him. The Silver Surfer refused, and Doom attacked him, but the Silver Surfer blasted him with a cosmic energy wave, healing Doom's body in the process (but not taking away his powers).
Doom made a deal with the American Military, who had the Fantastic Four work with him to capture the Silver Surfer. However, Doom's part in this was a setup so he could obtain the Silver Surfer's board. He took the board and killed many US military personnel. The Fantastic Four fought against him, but Doom mortally wounded Sue, although she, Reed, and Ben were able to transfer their power to Johnny due to the Silver Surfer giving him the ability to switch powers with anyone he touches. Using the combined powers of the Fantastic Four, Johnny fought Doom, ending with Ben hitting him with a crane, sending him falling into the water at the harbor where he sank to watery depths to his presumed death.