Physicist Reed Richards is convinced evolution was triggered millions of years ago on Earth by clouds of cosmic energy in space, and has calculated that one of these clouds is soon going to pass near Earth. Together with his friend, astronaut Ben Grimm, Reed convinces Victor Von Doom, his former classmate at MIT and now CEO of Von Doom Industries, to allow him access to his privately owned space station to test the effects of a biological sample of exposure to the cloud. Doom agrees in exchange for control over the experiment and a majority of the profits from whatever benefits it brings. Reed brings aboard his ex-girlfriend and Von Doom's chief genetics researcher Sue Storm and her ex-astronaut younger brother Johnny Storm.
The quintet travels to outer space to observe the cosmic energy clouds, but Reed miscalculates and the clouds materialize ahead of schedule. Reed and the Storms leave the shielded station to rescue Ben, who had gone on a spacewalk to place the samples. Ben receives full exposure in outer space, while the others receive a more limited dose within the station. Back home they soon develop superpowers: Reed can stretch his body like rubber, Susan can become invisible and generate force shields, Johnny Storm can engulf himself in fire and fly unaided, and Ben becomes a rock-like creature with superhuman strength and durability. Meanwhile, Von Doom faces a backlash from his stockholders because of the publicity from the space mission, and has a scar on his face that came from an exploding control console on the station.
Ben's fiancee Debbie cannot handle his new appearance and leaves him. Ben goes to brood on the Brooklyn Bridge and accidentally causes a traffic pileup while preventing a man from committing suicide. Ben, Reed and the Storms use their various abilities to contain the damage and prevent harm. The media dubs them the Fantastic Four. They move into Reed's lab in the Baxter Building to study their abilities and seek a way to return Grimm to normal. Von Doom, himself mutating, offers his support but blames Reed for the failure of the spaceflight, which has lost him his company.
Reed tells the group he will construct a machine to recreate the storm and reverse its effects on them, but warns it could possibly accelerate them instead. Meanwhile, Von Doom's arm has become organic metal, giving him superhuman strength allowing him to produce bolts of electricity, and he begins plotting revenge. He drives a wedge between Ben and Reed, who has rekindled his relationship with Susan Storm. Using the machine, Von Doom restores Ben to human form, while accelerating Von Doom's condition, causing much of his body to turn to metal. Von Doom knocks the human Grimm unconscious and captures Reed.
Now calling himself Doctor Doom, he puts on a metallic mask and a cloak to hide his disfigurement, Doom then tortures Reed and fires a heatseeking missile at the Baxter Building in an unsuccessful attempt to defeat Johnny. Sue confronts Doom but is outmatched. Ben arrives to assist her, transformed into the Thing again by reusing the machine (speaking his signature line, "It's clobberin' time!" which used in Johnny's action figure). The battle then gets spilled into the streets and The Storms combine their powers to wrap Doom in an inferno of intense heat, while Ben and Reed douse him with cold water, inducing thermal shock and freezing Doom in the place, defeating him and preventing the destruction of the city. In an epilogue, Grimm informs Reed that he has accepted his condition with the help of Alicia Masters, a blind artist for whom he has developed feelings, and the team embraces its role as superheroes. Reed proposes marriage to Sue, who accepts. Meanwhile, Doom's statue-like remains are being transported back to his homeland of Latveria when the dock master's electronic manifest briefly undergoes electromagnetic interference, suggesting that Doom is still alive.

Question: Why does Ben change back into the Thing?
Answer: To use his powers against Dr. Doom
Alternative answer: to rescue Sue